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/providers/chat_provider.dart

@@ -90,6 +90,7 @@ class MessagesNotifier extends FamilyAsyncNotifier<List<Message>, int> {
       await for (final chunk in repo.streamGeneration(convId)) {
         streamedContent = true;
         final msgs = state.requireValue;
+        if (msgs.isEmpty) continue;
         final updated = msgs.last.copyWith(content: msgs.last.content + chunk);
         state = AsyncData([...msgs.sublist(0, msgs.length - 1), updated]);
       }