Fix bugs found in full code audit

- Edit screens (note, task): cache _initFuture in initState instead of
  calling _loadExisting() from FutureBuilder on every rebuild. Prevents
  race condition where multiple concurrent loads could overwrite edits.
  Also removes unnecessary _loaded flag and simplifies the pattern.

- Note editor: remove onChanged: (_) => setState((){}) on the body
  TextField — triggered a full rebuild on every keystroke for no reason.

- SSE streaming: use utf8.decode(chunk, allowMalformed: true) so a
  malformed byte sequence from the server skips rather than throwing an
  unhandled FormatException that escaped all catch blocks.

- Chat provider: guard msgs.isEmpty before accessing msgs.last during
  SSE chunk processing to prevent a potential index error on empty state.

- Offline queue drain: check mounted before and after each async gap in
  _drainQueue so ref is never accessed on a disposed widget.

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

lib/screens/tasks/task_edit_screen.dart

@@ -23,7 +23,16 @@ class _TaskEditScreenState extends ConsumerState<TaskEditScreen> {
   TaskPriority _priority = TaskPriority.medium;
   DateTime? _dueDate;
   bool _saving = false;
-  bool _loaded = false;
+
+  // Future is created once in initState so FutureBuilder never restarts it.
+  late final Future<void> _initFuture;
+
+  @override
+  void initState() {
+    super.initState();
+    _initFuture =
+        widget.taskId != null ? _loadExisting() : Future.value();
+  }
 
   @override
   void dispose() {
@@ -33,17 +42,13 @@ class _TaskEditScreenState extends ConsumerState<TaskEditScreen> {
   }
 
   Future<void> _loadExisting() async {
-    if (_loaded || widget.taskId == null) {
-      _loaded = true;
-      return;
-    }
-    final task = await ref.read(tasksRepositoryProvider).getOne(widget.taskId!);
+    final task =
+        await ref.read(tasksRepositoryProvider).getOne(widget.taskId!);
     _titleController.text = task.title;
     _descController.text = task.description ?? '';
     _status = task.status;
     _priority = task.priority;
     _dueDate = task.dueDate;
-    _loaded = true;
   }
 
   Future<void> _save() async {
@@ -117,7 +122,7 @@ class _TaskEditScreenState extends ConsumerState<TaskEditScreen> {
   @override
   Widget build(BuildContext context) {
     return FutureBuilder(
-      future: _loadExisting(),
+      future: _initFuture,
       builder: (context, snapshot) {
         return Scaffold(
           appBar: AppBar(