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.