feat(tabs,app): tab id handles, fixed-width tabs-lift, slim shell + full-height sidebar

Tabs:
- short-handle ID badge per tab (shortest unique conversationId prefix, min 4)
- fixed-width (w-48) tabs with tabs-lift folder borders

Shell (composition root):
- drop the Dispatch title bar; tabs sit at the very top with a 5px gap
- big faded "Dispatch" watermark centered on an empty chat
- collapsible right sidebar (empty shell) spanning full window height: a
  permanently right-pinned hamburger in the tab row toggles it; in-flow push
  that shrinks the whole left column (tabs included) at >=lg, overlay + backdrop
  below lg; open-by-default on wide / closed on narrow
- main is overflow-hidden with a min-w-0 shrink chain; app.css pins
  html/body/#app height + body overflow hidden so the page never overflows

7 files changed, 239 insertions, 53 deletions

diff --git a/src/app.css b/src/app.css
index 37131d3..5db1f25 100644
--- a/src/app.css
+++ b/src/app.css
@@ -6,3 +6,15 @@
 @plugin "daisyui" {
 	themes: dracula --default;
 }
+
+/* App shell fills the viewport and never scrolls/overflows at the page level —
+   the inner regions (tab strip, chat transcript) own their own scrolling. */
+html,
+body,
+#app {
+	height: 100%;
+}
+
+body {
+	overflow: hidden;
+}
diff --git a/src/app/App.svelte b/src/app/App.svelte
index 857a1e5..4ee071d 100644
--- a/src/app/App.svelte
+++ b/src/app/App.svelte
@@ -7,6 +7,12 @@
 
 	let { store }: { store: AppStore } = $props();
 
+	// Right sidebar: open by default on wide screens (pushes the chat aside),
+	// closed by default on narrow screens (overlays the chat). Initial state is
+	// derived from the viewport width once; the hamburger toggles it thereafter.
+	const WIDE_BREAKPOINT = 1024; // Tailwind `lg`
+	let sidebarOpen = $state(typeof window !== "undefined" ? window.innerWidth >= WIDE_BREAKPOINT : true);
+
 	function handleSelect(surfaceId: string) {
 		store.select(surfaceId);
 	}
@@ -24,34 +30,59 @@
 	}
 </script>
 
-<main class="flex h-screen flex-col">
-	<div class="flex items-center justify-between border-b border-base-300 px-4 py-2">
-		<h1 class="text-lg font-bold">Dispatch</h1>
-	</div>
-
-	{#if store.lastError}
-		<div role="alert" class="alert alert-error mx-4 mt-2">
-			<strong>Error:</strong>
-			{store.lastError.message}
+<main class="relative flex h-screen overflow-hidden">
+	<!-- LEFT: everything except the sidebar. The full-height sidebar is a sibling
+	     (below), so opening it shrinks this ENTIRE column — tab row included, which
+	     slides the hamburger left. -->
+	<div class="flex min-w-0 flex-1 flex-col overflow-hidden pt-[5px]">
+		<!-- Tab row: the tab strip fills + scrolls internally (flex-1 min-w-0), with
+		     a permanently seated hamburger pinned to the far right. -->
+		<div class="flex min-w-0 items-center">
+			<TabBar
+				tabs={store.tabs}
+				activeConversationId={store.activeConversationId}
+				onSelect={(id) => store.selectTab(id)}
+				onClose={(id) => store.closeTab(id)}
+				onNewDraft={() => store.newDraft()}
+			/>
+			<button
+				class="btn btn-square btn-ghost btn-sm mx-1 shrink-0"
+				aria-label="Toggle sidebar"
+				aria-expanded={sidebarOpen}
+				onclick={() => (sidebarOpen = !sidebarOpen)}
+			>
+				<svg
+					xmlns="http://www.w3.org/2000/svg"
+					fill="none"
+					viewBox="0 0 24 24"
+					stroke-width="2"
+					stroke="currentColor"
+					class="size-5"
+					aria-hidden="true"
+				>
+					<path
+						stroke-linecap="round"
+						stroke-linejoin="round"
+						d="M3.75 6.75h16.5M3.75 12h16.5M3.75 17.25h16.5"
+					/>
+				</svg>
+			</button>
 		</div>
-	{/if}
 
-	{#if store.activeChat.error}
-		<div role="alert" class="alert alert-warning mx-4 mt-2">
-			<strong>Chat error:</strong>
-			{store.activeChat.error}
-		</div>
-	{/if}
+		{#if store.lastError}
+			<div role="alert" class="alert alert-error mx-4 mt-2">
+				<strong>Error:</strong>
+				{store.lastError.message}
+			</div>
+		{/if}
 
-	<TabBar
-		tabs={store.tabs}
-		activeConversationId={store.activeConversationId}
-		onSelect={(id) => store.selectTab(id)}
-		onClose={(id) => store.closeTab(id)}
-		onNewDraft={() => store.newDraft()}
-	/>
+		{#if store.activeChat.error}
+			<div role="alert" class="alert alert-warning mx-4 mt-2">
+				<strong>Chat error:</strong>
+				{store.activeChat.error}
+			</div>
+		{/if}
 
-	<div class="flex flex-1 flex-col overflow-hidden">
 		<div class="flex items-center gap-2 px-4 py-2">
 			<ModelSelector
 				models={store.models}
@@ -60,37 +91,76 @@
 			/>
 		</div>
 
-		<div class="flex-1 overflow-y-auto">
+		<div class="relative min-w-0 flex-1 overflow-y-auto">
 			{#key store.activeConversationId}
 				<ChatView chunks={store.activeChat.chunks} turnMetrics={store.activeChat.turnMetrics} />
 			{/key}
+			{#if store.activeChat.chunks.length === 0}
+				<div
+					class="pointer-events-none absolute inset-0 flex items-center justify-center"
+					aria-hidden="true"
+				>
+					<span class="select-none text-4xl font-bold opacity-10">Dispatch</span>
+				</div>
+			{/if}
 		</div>
 
 		<Composer onSend={handleSend} />
+
+		{#if store.catalog.length > 0}
+			<section class="border-t border-base-300 p-4">
+				<h2 class="mb-2 text-sm font-semibold">Surfaces</h2>
+				<div class="flex flex-wrap gap-2">
+					{#each store.catalog as entry (entry.id)}
+						<button
+							class="btn btn-sm"
+							class:btn-active={entry.id === store.selectedId}
+							aria-current={entry.id === store.selectedId ? "true" : undefined}
+							onclick={() => handleSelect(entry.id)}
+						>
+							{entry.title}
+							<span class="text-xs opacity-60">({entry.region})</span>
+						</button>
+					{/each}
+				</div>
+			</section>
+		{/if}
+
+		{#if store.selectedSpec}
+			<section class="border-t border-base-300 p-4">
+				<SurfaceView spec={store.selectedSpec} onInvoke={handleInvoke} />
+			</section>
+		{/if}
 	</div>
 
-	{#if store.catalog.length > 0}
-		<section class="border-t border-base-300 p-4">
-			<h2 class="mb-2 text-sm font-semibold">Surfaces</h2>
-			<div class="flex flex-wrap gap-2">
-				{#each store.catalog as entry (entry.id)}
-					<button
-						class="btn btn-sm"
-						class:btn-active={entry.id === store.selectedId}
-						aria-current={entry.id === store.selectedId ? "true" : undefined}
-						onclick={() => handleSelect(entry.id)}
-					>
-						{entry.title}
-						<span class="text-xs opacity-60">({entry.region})</span>
-					</button>
-				{/each}
-			</div>
-		</section>
-	{/if}
+	<!-- Full-height right sidebar. On wide screens (`lg:relative`) it is in-flow, so
+	     opening it shrinks the whole left column (push). Below `lg` it overlays
+	     (`max-lg:absolute`, full height) with a backdrop. -->
+	<aside
+		class="flex shrink-0 flex-col overflow-x-hidden transition-[width] duration-300 ease-out max-lg:absolute max-lg:inset-y-0 max-lg:right-0 max-lg:z-30 lg:relative"
+		class:w-80={sidebarOpen}
+		class:w-0={!sidebarOpen}
+	>
+		<div
+			class="flex h-full w-80 flex-col gap-2 overflow-y-auto border-l border-base-300 bg-base-100 p-3 transition-transform duration-300 ease-out"
+			style="transform: translateX({sidebarOpen ? '0' : '100%'})"
+		>
+			<h2 class="text-sm font-semibold opacity-60">Sidebar</h2>
+		</div>
+	</aside>
 
-	{#if store.selectedSpec}
-		<section class="border-t border-base-300 p-4">
-			<SurfaceView spec={store.selectedSpec} onInvoke={handleInvoke} />
-		</section>
+	<!-- Backdrop: only on narrow screens (overlay mode), click to close. -->
+	{#if sidebarOpen}
+		<!-- svelte-ignore a11y_no_static_element_interactions -->
+		<div
+			class="fixed inset-0 z-20 bg-black/30 lg:hidden"
+			role="button"
+			tabindex="0"
+			aria-label="Close sidebar"
+			onclick={() => (sidebarOpen = false)}
+			onkeydown={(e) => {
+				if (e.key === "Escape" || e.key === "Enter") sidebarOpen = false;
+			}}
+		></div>
 	{/if}
 </main>
diff --git a/src/features/tabs/index.ts b/src/features/tabs/index.ts
index 835788a..50de62a 100644
--- a/src/features/tabs/index.ts
+++ b/src/features/tabs/index.ts
@@ -5,10 +5,12 @@ export {
 	createTab,
 	deriveTitle,
 	initialState,
+	MIN_HANDLE_LENGTH,
 	newDraft,
 	selectTab,
 	setModel,
 	setTitle,
+	shortHandle,
 } from "./tabs";
 export type { TabsStorage, TabsStore } from "./tabs-store.svelte";
 export { createTabsStore } from "./tabs-store.svelte";
diff --git a/src/features/tabs/tabs.test.ts b/src/features/tabs/tabs.test.ts
index 5effa9b..3c2a8c2 100644
--- a/src/features/tabs/tabs.test.ts
+++ b/src/features/tabs/tabs.test.ts
@@ -7,10 +7,12 @@ import {
 	deriveTitle,
 	initialState,
 	isStuckToEnd,
+	MIN_HANDLE_LENGTH,
 	newDraft,
 	selectTab,
 	setModel,
 	setTitle,
+	shortHandle,
 } from "./tabs";
 
 const tab = (conversationId: string, model = "default", title = "Chat"): Tab => ({
@@ -213,3 +215,34 @@ describe("isStuckToEnd", () => {
 		expect(isStuckToEnd({ scrollLeft: 499, clientWidth: 500, scrollWidth: 1000 })).toBe(false);
 	});
 });
+
+describe("shortHandle", () => {
+	it("uses the minimum length when the id is unique", () => {
+		const h = shortHandle("3f9a1b2c-aaaa", ["3f9a1b2c-aaaa", "7c2d-bbbb"]);
+		expect(h).toBe("3f9a");
+		expect(h.length).toBe(MIN_HANDLE_LENGTH);
+	});
+
+	it("grows the prefix until unique among open tabs", () => {
+		// two ids share the first 5 chars → handle grows to 6 to disambiguate
+		expect(shortHandle("abcde1-xxxx", ["abcde1-xxxx", "abcde2-yyyy"])).toBe("abcde1");
+		expect(shortHandle("abcde2-yyyy", ["abcde1-xxxx", "abcde2-yyyy"])).toBe("abcde2");
+	});
+
+	it("shrinks back to the minimum when the colliding sibling is gone", () => {
+		expect(shortHandle("abcde1-xxxx", ["abcde1-xxxx"])).toBe("abcd");
+	});
+
+	it("ignores the id itself when present in the list", () => {
+		expect(shortHandle("deadbeef", ["deadbeef"])).toBe("dead");
+	});
+
+	it("returns the whole id when shorter than the minimum length", () => {
+		expect(shortHandle("ab", ["ab", "cd"])).toBe("ab");
+	});
+
+	it("falls back to the full id when one id is a prefix of another", () => {
+		// "abcd" is a prefix of "abcd1234" → no unique shorter prefix exists for it
+		expect(shortHandle("abcd", ["abcd", "abcd1234"])).toBe("abcd");
+	});
+});
diff --git a/src/features/tabs/tabs.ts b/src/features/tabs/tabs.ts
index a4db6f3..61ae58e 100644
--- a/src/features/tabs/tabs.ts
+++ b/src/features/tabs/tabs.ts
@@ -92,3 +92,22 @@ export function deriveTitle(message: string, max: number = DEFAULT_MAX_TITLE_LEN
 	if (trimmed.length <= max) return trimmed;
 	return `${trimmed.slice(0, max)}\u2026`;
 }
+
+/** Minimum length of a tab handle (git-style short id). */
+export const MIN_HANDLE_LENGTH = 4;
+
+/**
+ * The short "handle" shown on a tab: the shortest prefix of `conversationId`
+ * (at least `MIN_HANDLE_LENGTH` chars) that is unique among all open tabs — a
+ * git-style short id. Grows by a char only when another open tab shares the
+ * prefix, and shrinks back when that sibling closes. Pure: the id + every open
+ * id in, the handle string out. (`allIds` may include `conversationId` itself.)
+ */
+export function shortHandle(conversationId: string, allIds: readonly string[]): string {
+	const others = allIds.filter((id) => id !== conversationId);
+	for (let len = MIN_HANDLE_LENGTH; len < conversationId.length; len++) {
+		const candidate = conversationId.slice(0, len);
+		if (!others.some((id) => id.startsWith(candidate))) return candidate;
+	}
+	return conversationId;
+}
diff --git a/src/features/tabs/ui.test.ts b/src/features/tabs/ui.test.ts
index 1ae18c8..6cd66bd 100644
--- a/src/features/tabs/ui.test.ts
+++ b/src/features/tabs/ui.test.ts
@@ -175,4 +175,39 @@ describe("TabBar", () => {
 		const newChat = screen.getByRole("button", { name: "New chat" });
 		expect(newChat).not.toHaveTextContent("New Chat");
 	});
+
+	it("renders a short-handle tab ID badge (shortest unique prefix) per tab", () => {
+		const tabs: readonly Tab[] = [
+			{ conversationId: "3f9a1b2c-1111", model: "m", title: "Alpha" },
+			{ conversationId: "7c2db4e5-2222", model: "m", title: "Beta" },
+		];
+		render(TabBar, {
+			props: {
+				tabs,
+				activeConversationId: "3f9a1b2c-1111",
+				onSelect: vi.fn(),
+				onClose: vi.fn(),
+				onNewDraft: vi.fn(),
+			},
+		});
+
+		expect(screen.getByText("3f9a")).toBeInTheDocument();
+		expect(screen.getByText("7c2d")).toBeInTheDocument();
+	});
+
+	it("renders fixed-width tabs", () => {
+		render(TabBar, {
+			props: {
+				tabs: sampleTabs,
+				activeConversationId: "c1",
+				onSelect: vi.fn(),
+				onClose: vi.fn(),
+				onNewDraft: vi.fn(),
+			},
+		});
+
+		for (const t of screen.getAllByRole("tab")) {
+			expect(t).toHaveClass("w-48");
+		}
+	});
 });
diff --git a/src/features/tabs/ui/TabBar.svelte b/src/features/tabs/ui/TabBar.svelte
index eb5b5e5..812a663 100644
--- a/src/features/tabs/ui/TabBar.svelte
+++ b/src/features/tabs/ui/TabBar.svelte
@@ -1,6 +1,6 @@
 <script lang="ts">
 	import type { Tab } from "../tabs";
-	import { isStuckToEnd } from "../tabs";
+	import { isStuckToEnd, shortHandle } from "../tabs";
 
 	let {
 		tabs,
@@ -23,6 +23,15 @@
 	let scrollEl = $state<HTMLDivElement>();
 	let stuck = $state(false);
 
+	// Git-style short handle (shortest unique prefix) per open tab — the visible
+	// "tab ID". Derived from the set of open conversation ids; pure helper.
+	const handles = $derived.by(() => {
+		const ids = tabs.map((t) => t.conversationId);
+		const map = new Map<string, string>();
+		for (const id of ids) map.set(id, shortHandle(id, ids));
+		return map;
+	});
+
 	function recompute(): void {
 		const el = scrollEl;
 		if (el === undefined) {
@@ -55,11 +64,11 @@
 	});
 </script>
 
-<div bind:this={scrollEl} class="overflow-x-auto border-b border-base-300">
-	<div class="tabs tabs-border min-w-max">
+<div bind:this={scrollEl} class="min-w-0 flex-1 overflow-x-auto">
+	<div class="tabs tabs-lift min-w-max">
 		{#each tabs as tab (tab.conversationId)}
 			<div
-				class="tab"
+				class="tab flex w-48 shrink-0 items-center gap-1.5"
 				class:tab-active={tab.conversationId === activeConversationId}
 				role="tab"
 				tabindex="0"
@@ -68,9 +77,15 @@
 					if (e.key === "Enter") onSelect(tab.conversationId);
 				}}
 			>
-				<span class="max-w-[120px] truncate">{tab.title}</span>
+				<span
+					class="shrink-0 rounded bg-base-300 px-1 py-0.5 font-mono text-[10px] leading-none text-base-content/60"
+					title="Tab ID"
+				>
+					{handles.get(tab.conversationId) ?? tab.conversationId}
+				</span>
+				<span class="min-w-0 flex-1 truncate text-left">{tab.title}</span>
 				<button
-					class="btn btn-ghost btn-xs ml-1"
+					class="btn btn-ghost btn-xs shrink-0"
 					aria-label="Close tab"
 					onclick={(e) => {
 						e.stopPropagation();