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.

2 files changed, 22 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;