feat(briefing): stall watchdog + pull-to-refresh; chat attaches to in-flight streams

Briefing chat now uses the same StreamIterator + 45s per-event timeout
as the main chat provider, so a dropped SSE socket no longer leaves
isBriefingStreaming stuck true. Adds refreshMessages() that unfreezes
state when the server-side message is complete, wired to a new
pull-to-refresh gesture and the app lifecycle-resume hook.

Chat provider gains attachToGeneration() — safe to call unconditionally
on screen init, so landing on a conversation that's already mid-stream
(e.g. the /news discuss button, which now creates a conv and auto-kicks
generation server-side) picks up live tokens instead of freezing on the
placeholder.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/chat/chat_screen.dart

@@ -44,6 +44,16 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
   void initState() {
     super.initState();
     WidgetsBinding.instance.addObserver(this);
+    // If we land on a conversation whose last assistant message is already
+    // mid-stream (e.g. the /news discuss button creates a conv and
+    // auto-kicks generation), attach to the running stream so the user sees
+    // live tokens instead of a frozen placeholder.
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted) return;
+      ref
+          .read(messagesProvider(widget.conversationId).notifier)
+          .attachToGeneration();
+    });
   }
 
   @override