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/components/StaleStreamBanner.tsx

@@ -0,0 +1,34 @@
+interface Props {
+  onRetry: () => void;
+  onDiscard: () => void;
+}
+
+// v1.12.3: shown when an assistant message has been 'streaming' for 60+
+// seconds without new tokens. Lives above ChatInput in ChatPane. Retry
+// discards the stuck row then resends the last user message; Discard just
+// clears the row and drops the dot to idle.
+export function StaleStreamBanner({ onRetry, onDiscard }: Props) {
+  return (
+    <div className="border border-amber-500/30 bg-amber-500/5 rounded-md p-3 mb-2 mx-4 flex items-center justify-between gap-2">
+      <span className="text-sm text-muted-foreground">
+        Previous response didn't complete.
+      </span>
+      <div className="flex gap-2">
+        <button
+          type="button"
+          onClick={onRetry}
+          className="text-xs px-2 py-1 rounded border border-border hover:bg-accent max-md:min-h-[44px] max-md:px-3"
+        >
+          Retry
+        </button>
+        <button
+          type="button"
+          onClick={onDiscard}
+          className="text-xs px-2 py-1 rounded border border-border hover:bg-accent max-md:min-h-[44px] max-md:px-3"
+        >
+          Discard
+        </button>
+      </div>
+    </div>
+  );
+}