feat(ai-chat): mark interrupted turns with a "stopped" notice

A turn that ends without a clean finish now shows a neutral marker, so an
interrupted answer is visible instead of trailing off silently. Errors keep
their existing red banner; this covers the aborted case.

- chat-stopped-notice.tsx: new neutral (gray) notice component
- chat-thread.tsx: live marker driven by useChat onFinish flags — distinguishes
  a manual Stop (isAbort) from a dropped connection (isDisconnect); cleared when
  the next turn streams; flushNext still runs only on a clean finish
- message-item.tsx: per-message marker in reopened history for finishReason
  'aborted' with no error (combined wording, since the server can't tell a
  manual Stop from a dropped connection)
- ai-chat.types.ts: add metadata.finishReason; rowToUiMessage now carries it
- en-US: three new strings

Frontend only — the server already persists partial work and finishReason and
replays it to the model on the next turn (continue, not restart).

apps/client/public/locales/en-US/translation.json

@@ -1154,6 +1154,9 @@
   "Queue message": "Queue message",
   "Remove queued message": "Remove queued message",
   "Stop": "Stop",
+  "Response stopped.": "Response stopped.",
+  "Connection lost — the answer was interrupted.": "Connection lost — the answer was interrupted.",
+  "Response stopped (manually or the connection dropped).": "Response stopped (manually or the connection dropped).",
   "Chat menu": "Chat menu",
   "No chats yet.": "No chats yet.",
   "Delete this chat?": "Delete this chat?",