feat(views,surface-host): Extensions sidebar view — auto-expanded surfaces + tables

views (new feature):
- pure panel-stack reducer + thin generic ViewSidebar (dropdown picker + add/remove),
  switches on view KIND, never a surface id

Extensions view (composition root):
- folds frontend modules + backend surfaces into one "Extensions" view
- frontend module list AGGREGATED from each feature's public `manifest` export
  (can't drift); no per-module version (FE features are internal to dispatch-web)
- surfaces are AUTO-SUBSCRIBED on catalog + rendered expanded (no catalog buttons)

surface-host:
- consecutive `stat` fields coalesce into one aligned label/value table (StatTable)
- generic custom-field renderer: dispatch on rendererId === "table" → SurfaceTable
  (pure parseTablePayload), so a backend `custom`/table field renders generically
- shared presentational components/Table.svelte (used by both, neither feature
  depends on the other)

store:
- auto-subscribe every catalog entry, unsubscribe vanished ones, re-subscribe all
  on reconnect; expose all received specs via `surfaces` (drops single-selection)

backend-handoff: CR-1 — emit Loaded Extensions as a custom/table field; notes
what's already covered FE-side (renderer shipped, stat-table fallback works).

2 files changed, 145 insertions, 0 deletions

diff --git a/src/features/views/ui/ViewSidebar.svelte b/src/features/views/ui/ViewSidebar.svelte
new file mode 100644
index 0000000..c4b466f
--- /dev/null
+++ b/src/features/views/ui/ViewSidebar.svelte
@@ -0,0 +1,87 @@
+<script lang="ts">
+	import { type Snippet, untrack } from "svelte";
+	import {
+		addPanel,
+		initialPanels,
+		type PanelsState,
+		removePanel,
+		selectKind,
+	} from "../logic/panels";
+
+	interface ViewKind {
+		readonly id: string;
+		readonly label: string;
+	}
+
+	let {
+		kinds,
+		content,
+		initial,
+	}: {
+		/** The view kinds offered in every panel's dropdown. */
+		kinds: readonly ViewKind[];
+		/** Renders a panel body for the given (non-null) view-kind id. */
+		content: Snippet<[string]>;
+		/** Optional seed of panel kinds; defaults to one panel of the first kind. */
+		initial?: readonly (string | null)[];
+	} = $props();
+
+	// Local UI composition state, owned by this unit and folded through the pure
+	// reducer — never reached from elsewhere (no ambient store). Seeded ONCE from
+	// the props (untrack makes that one-time read explicit, not reactive).
+	let state = $state<PanelsState>(
+		untrack(() => initialPanels(initial ?? [kinds[0]?.id ?? null])),
+	);
+</script>
+
+<div class="flex min-h-0 flex-col gap-2">
+	{#each state.panels as panel, idx (panel.id)}
+		<div class="flex flex-col rounded-lg bg-base-200 p-3">
+			<div class="flex items-center gap-1">
+				<select
+					class="select select-bordered select-sm flex-1"
+					aria-label="Select a view"
+					value={panel.kind ?? ""}
+					onchange={(e) => {
+						const v = e.currentTarget.value;
+						state = selectKind(state, panel.id, v === "" ? null : v);
+					}}
+				>
+					<option value="" disabled>Select a view</option>
+					{#each kinds as kind (kind.id)}
+						<option value={kind.id}>{kind.label}</option>
+					{/each}
+				</select>
+				{#if idx > 0}
+					<button
+						type="button"
+						class="btn btn-square btn-ghost btn-sm shrink-0"
+						aria-label="Remove view"
+						onclick={() => {
+							state = removePanel(state, panel.id);
+						}}
+					>
+						✕
+					</button>
+				{/if}
+			</div>
+
+			{#if panel.kind !== null}
+				<div class="mt-2">
+					{@render content(panel.kind)}
+				</div>
+			{/if}
+		</div>
+	{/each}
+
+	<button
+		type="button"
+		class="btn w-full border-none bg-base-200 text-lg hover:bg-base-300"
+		aria-label="Add view"
+		onclick={() => {
+			state = addPanel(state);
+		}}
+	>
+		+
+	</button>
+</div>
diff --git a/src/features/views/ui/ViewSidebar.test.ts b/src/features/views/ui/ViewSidebar.test.ts
new file mode 100644
index 0000000..8a0049c
--- /dev/null
+++ b/src/features/views/ui/ViewSidebar.test.ts
@@ -0,0 +1,58 @@
+import { render, screen } from "@testing-library/svelte";
+import userEvent from "@testing-library/user-event";
+import { createRawSnippet } from "svelte";
+import { describe, expect, it } from "vitest";
+import ViewSidebar from "./ViewSidebar.svelte";
+
+const kinds = [
+	{ id: "surfaces", label: "Surfaces" },
+	{ id: "tasks", label: "Tasks" },
+];
+
+// A raw snippet that echoes the kind it was rendered for, so tests can assert
+// which view-kind content each panel shows.
+const content = createRawSnippet<[string]>((kind) => ({
+	render: () => `<div data-testid="view-content">kind:${kind()}</div>`,
+}));
+
+describe("ViewSidebar", () => {
+	it("opens one panel seeded with the first kind and renders its content", () => {
+		render(ViewSidebar, { props: { kinds, content } });
+		expect(screen.getAllByRole("combobox")).toHaveLength(1);
+		expect(screen.getByTestId("view-content")).toHaveTextContent("kind:surfaces");
+	});
+
+	it("the first panel has no remove button", () => {
+		render(ViewSidebar, { props: { kinds, content } });
+		expect(screen.queryByRole("button", { name: "Remove view" })).toBeNull();
+	});
+
+	it("the add button appends a new empty panel", async () => {
+		const user = userEvent.setup();
+		render(ViewSidebar, { props: { kinds, content } });
+		await user.click(screen.getByRole("button", { name: "Add view" }));
+		expect(screen.getAllByRole("combobox")).toHaveLength(2);
+		// the new panel is empty → only the first panel renders content
+		expect(screen.getAllByTestId("view-content")).toHaveLength(1);
+	});
+
+	it("non-first panels can be removed", async () => {
+		const user = userEvent.setup();
+		render(ViewSidebar, { props: { kinds, content } });
+		await user.click(screen.getByRole("button", { name: "Add view" }));
+		const removeButtons = screen.getAllByRole("button", { name: "Remove view" });
+		expect(removeButtons).toHaveLength(1);
+		const target = removeButtons[0];
+		if (target === undefined) throw new Error("expected a remove button");
+		await user.click(target);
+		expect(screen.getAllByRole("combobox")).toHaveLength(1);
+	});
+
+	it("selecting a kind renders that kind's content", async () => {
+		const user = userEvent.setup();
+		render(ViewSidebar, { props: { kinds, content, initial: [null] } });
+		expect(screen.queryByTestId("view-content")).toBeNull();
+		await user.selectOptions(screen.getByRole("combobox"), "tasks");
+		expect(screen.getByTestId("view-content")).toHaveTextContent("kind:tasks");
+	});
+});