feat(chat): stop generation button — abort without closing

Consume the stop-generation handoff (no version bumps, no new types).

- App store: stopGeneration() → POST /conversations/:id/stop (fire-and-forget)
- Composer: stop button (square, error color) visible only while generating,
  next to the send/queue button
- Existing event flow handles the rest: done with reason 'aborted' clears
  generating; conversation.statusChanged: idle updates the tab spinner

686 tests green.

3 files changed, 42 insertions, 0 deletions

diff --git a/src/app/App.svelte b/src/app/App.svelte
index 350db49..3024a44 100644
--- a/src/app/App.svelte
+++ b/src/app/App.svelte
@@ -195,6 +195,10 @@
 		store.queueMessage(text);
 	}
 
+	function handleStop() {
+		store.stopGeneration();
+	}
+
 	function handleSelectModel(model: string) {
 		store.selectModel(model);
 	}
@@ -372,6 +376,7 @@
 		<Composer
 			onSend={handleSend}
 			onQueue={handleQueue}
+			onStop={handleStop}
 			contextSize={store.activeChat.currentContextSize}
 			status={store.activeChat.error
 				? "error"
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 1359fcd..f212920 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -142,6 +142,13 @@ export interface AppStore {
 	 */
 	compactNow(keepLastN?: number): Promise<CompactResult | null>;
 	/**
+	 * Stop an in-flight generation (`POST /conversations/:id/stop`). Aborts the
+	 * turn without closing the conversation — partial messages are persisted, the
+	 * turn seals with `reason: "aborted"`, and the conversation goes `active → idle`.
+	 * Returns null when no conversation is focused.
+	 */
+	stopGeneration(): void;
+	/**
 	 * The workspace conversation's auto-compact threshold (tokens). `0` = disabled
 	 * (manual only); a positive number = auto-compact triggers when the last
 	 * turn's input tokens exceed it. Seeded from the backend on focus change.
@@ -934,6 +941,16 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 			}
 		},
 
+		stopGeneration(): void {
+			const conversationId = tabsStore.activeConversationId;
+			if (conversationId === null) return;
+			void fetchImpl(`${httpBase}/conversations/${encodeURIComponent(conversationId)}/stop`, {
+				method: "POST",
+			}).catch(() => {
+				// Non-fatal — the existing event flow handles the turn settle.
+			});
+		},
+
 		async compactNow(keepLastN?: number): Promise<CompactResult | null> {
 			const conversationId = tabsStore.activeConversationId;
 			if (conversationId === null) return null;
diff --git a/src/features/chat/ui/Composer.svelte b/src/features/chat/ui/Composer.svelte
index d519efc..5952eca 100644
--- a/src/features/chat/ui/Composer.svelte
+++ b/src/features/chat/ui/Composer.svelte
@@ -9,6 +9,7 @@
 	let {
 		onSend,
 		onQueue,
+		onStop,
 		contextSize = undefined,
 		status = "idle",
 	}: {
@@ -20,6 +21,8 @@
 		 * used regardless (tests / non-steering contexts).
 		 */
 		onQueue?: (text: string) => void;
+		/** Stop the in-flight generation (`POST /conversations/:id/stop`). */
+		onStop?: () => void;
 		// Current context occupancy (latest turn's contextSize), or `undefined`
 		// when unknown — the status bar then shows "— tokens", never 0%.
 		contextSize?: number | undefined;
@@ -110,6 +113,23 @@
 		<button class="btn btn-primary w-20 shrink-0" type="submit" disabled={!hasText}>
 			{submitLabel}
 		</button>
+		{#if status === "running" && onStop}
+			<button
+				class="btn btn-error btn-square shrink-0"
+				type="button"
+				aria-label="Stop generation"
+				onclick={onStop}
+			>
+				<svg
+					xmlns="http://www.w3.org/2000/svg"
+					viewBox="0 0 24 24"
+					fill="currentColor"
+					class="h-4 w-4"
+				>
+					<rect x="6" y="6" width="12" height="12" rx="2"></rect>
+				</svg>
+			</button>
+		{/if}
 	</div>
 
 	<!-- Bottom status bar: status icon · context-window fill · token count -->