v1.12.3: stale-stream banner with Retry/Discard

When an assistant message sits status='streaming' with no token activity
for 60+ seconds, the chat shows a banner above the input offering Retry
or Discard. Both clear the stale row via a new backend endpoint
POST /api/chats/:id/discard_stale that updates status='failed' and
publishes chat_status='idle'.

Closes the UX gap that caused the 2026-05-21 debugging spiral —
slow streams and dead streams now look different to the user.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/api/client.ts

@@ -180,6 +180,11 @@ export const api = {
       request<{ ok: true }>(`/api/chats/${chatId}/compact`, { method: 'POST' }),
     stop: (chatId: string) =>
       request<{ stopped: boolean }>(`/api/chats/${chatId}/stop`, { method: 'POST' }),
+    discardStale: (chatId: string, messageId: string) =>
+      request<Message>(`/api/chats/${chatId}/discard_stale`, {
+        method: 'POST',
+        body: JSON.stringify({ message_id: messageId }),
+      }),
     forceSend: (chatId: string, content: string) =>
       request<{ user_message_id: string; assistant_message_id: string }>(
         `/api/chats/${chatId}/force_send`,