feat(chat): stall watchdog, pull-to-refresh, and unfreeze on refresh

Mobile chat occasionally froze with the input disabled and nothing
progressing — the underlying cause is SSE sockets dropping silently on
mobile network handoffs / proxy idle timeouts. Dio never observes the
close, so the send loop's `await for` hangs indefinitely with
isStreaming=true.

Three changes:
- Wrap the SSE stream in a StreamIterator with a 45s per-event timeout
  as a stall watchdog. On timeout, break out and let the polling pass
  reconcile state from the server.
- Add pull-to-refresh on the chat message list and an AppBar refresh
  button with inline spinner feedback. Works on both empty and
  populated states.
- MessagesNotifier.refresh() now also clears isStreaming /
  streamingStatus when the server reports the latest assistant message
  as complete — so a stuck UI unfreezes the moment the user pulls down
  or taps refresh, even if the original SSE loop is still hanging.

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

lib/providers/chat_provider.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../data/models/conversation.dart';
@@ -93,10 +95,27 @@ class MessagesNotifier extends AsyncNotifier<List<Message>> {
   }
 
   /// Re-fetch messages without clearing the current list (no flicker).
+  ///
+  /// Also unfreezes the UI if streaming state got stuck true — this happens
+  /// when an SSE connection dies silently (mobile network handoff, app
+  /// backgrounded mid-stream, reverse proxy dropping idle sockets) and the
+  /// send loop never observes a close. If the server-side message is already
+  /// done, we clear `isStreamingProvider` so the input unlocks.
   Future<void> refresh() async {
     final (_, messages) =
         await ref.read(chatRepositoryProvider).getMessages(_convId);
     state = AsyncData(messages);
+    Message? lastAssistant;
+    for (var i = messages.length - 1; i >= 0; i--) {
+      if (messages[i].role == MessageRole.assistant) {
+        lastAssistant = messages[i];
+        break;
+      }
+    }
+    if (lastAssistant != null && lastAssistant.status != 'generating') {
+      ref.read(isStreamingProvider(_convId).notifier).state = false;
+      ref.read(streamingStatusProvider(_convId).notifier).state = '';
+    }
   }
 
   Future<void> sendMessage(String content) async {
@@ -129,9 +148,19 @@ class MessagesNotifier extends AsyncNotifier<List<Message>> {
     }
 
     // ── Step 2: Stream the response (best effort — silent on failure). ──
+    //
+    // We use a StreamIterator with a per-event timeout as a stall watchdog.
+    // Mobile networks occasionally drop SSE sockets silently: the TCP
+    // connection is half-closed, Dio never sees the close, and `await for`
+    // hangs forever with `isStreaming=true`, freezing the input. If no
+    // event arrives within the watchdog window we bail out and let the
+    // polling pass below reconcile state from the server.
+    const stallTimeout = Duration(seconds: 45);
     bool streamedContent = false;
+    final iter = StreamIterator(repo.streamGeneration(convId));
     try {
-      await for (final event in repo.streamGeneration(convId)) {
+      while (await iter.moveNext().timeout(stallTimeout)) {
+        final event = iter.current;
         if (event is ChatTextChunk) {
           streamedContent = true;
           ref.read(streamingStatusProvider(convId).notifier).state = '';
@@ -157,8 +186,13 @@ class MessagesNotifier extends AsyncNotifier<List<Message>> {
           state = AsyncData([...msgs.sublist(0, msgs.length - 1), updated]);
         }
       }
+    } on TimeoutException {
+      // Stall watchdog — no SSE event for stallTimeout. Fall through to
+      // polling so the UI eventually unfreezes even if the socket is dead.
     } catch (_) {
       // SSE failed — fall through to the polling reload below.
+    } finally {
+      await iter.cancel();
     }
 
     // ── Step 3: Poll the API until we have a completed assistant response.

lib/screens/chat/chat_screen.dart

@@ -20,6 +20,25 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
     with WidgetsBindingObserver {
   final _controller = TextEditingController();
   final _scrollController = ScrollController();
+  bool _refreshing = false;
+
+  Future<void> _refreshMessages() async {
+    if (_refreshing) return;
+    setState(() => _refreshing = true);
+    try {
+      await ref
+          .read(messagesProvider(widget.conversationId).notifier)
+          .refresh();
+    } catch (_) {
+      if (mounted) {
+        ScaffoldMessenger.of(context).showSnackBar(
+          const SnackBar(content: Text('Could not refresh messages.')),
+        );
+      }
+    } finally {
+      if (mounted) setState(() => _refreshing = false);
+    }
+  }
 
   @override
   void initState() {
@@ -136,6 +155,19 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
     return Scaffold(
       appBar: AppBar(
         title: Text(convTitle?.isNotEmpty == true ? convTitle! : 'Chat'),
+        actions: [
+          IconButton(
+            tooltip: 'Refresh',
+            onPressed: _refreshing ? null : _refreshMessages,
+            icon: _refreshing
+                ? const SizedBox(
+                    width: 18,
+                    height: 18,
+                    child: CircularProgressIndicator(strokeWidth: 2),
+                  )
+                : const Icon(Icons.refresh),
+          ),
+        ],
       ),
       body: Column(
         children: [
@@ -147,12 +179,21 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
                 child: Text('Could not load messages.'),
               ),
               data: (messages) {
-                if (messages.isEmpty) {
-                  return const Center(
-                      child: Text('Send a message to start.'));
-                }
-                return ListView.builder(
+                return RefreshIndicator(
+                  onRefresh: _refreshMessages,
+                  child: messages.isEmpty
+                      ? ListView(
+                          // Needs to be scrollable for RefreshIndicator to
+                          // fire on empty state — plain Center won't work.
+                          physics: const AlwaysScrollableScrollPhysics(),
+                          children: const [
+                            SizedBox(height: 240),
+                            Center(child: Text('Send a message to start.')),
+                          ],
+                        )
+                      : ListView.builder(
                           controller: _scrollController,
+                          physics: const AlwaysScrollableScrollPhysics(),
                           padding: const EdgeInsets.symmetric(
                               horizontal: 8, vertical: 12),
                           itemCount: messages.length,
@@ -163,6 +204,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
                                 ? streamingStatus
                                 : '',
                           ),
+                        ),
                 );
               },
             ),