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.

1 files changed, 5 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"