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/notes/note_edit_screen.dart

@@ -20,7 +20,16 @@ class _NoteEditScreenState extends ConsumerState<NoteEditScreen> {
   final _contentController = TextEditingController();
   bool _preview = false;
   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.noteId != null ? _loadExisting() : Future.value();
+  }
 
   @override
   void dispose() {
@@ -30,15 +39,10 @@ class _NoteEditScreenState extends ConsumerState<NoteEditScreen> {
   }
 
   Future<void> _loadExisting() async {
-    if (_loaded || widget.noteId == null) {
-      _loaded = true;
-      return;
-    }
     final note =
         await ref.read(notesRepositoryProvider).getOne(widget.noteId!);
     _titleController.text = note.title;
     _contentController.text = note.body;
-    _loaded = true;
   }
 
   Future<void> _delete() async {
@@ -97,7 +101,7 @@ class _NoteEditScreenState extends ConsumerState<NoteEditScreen> {
   @override
   Widget build(BuildContext context) {
     return FutureBuilder(
-      future: _loadExisting(),
+      future: _initFuture,
       builder: (context, snapshot) {
         return Scaffold(
           appBar: AppBar(
@@ -159,7 +163,6 @@ class _NoteEditScreenState extends ConsumerState<NoteEditScreen> {
                                 expands: true,
                                 keyboardType: TextInputType.multiline,
                                 textAlignVertical: TextAlignVertical.top,
-                                onChanged: (_) => setState(() {}),
                               ),
                             ),
                     ),