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/app.dart

@@ -245,12 +245,15 @@ class _QuickCaptureBarState extends ConsumerState<_QuickCaptureBar> {
   }
 
   Future<void> _drainQueue() async {
+    if (!mounted) return;
     final queue = ref.read(captureQueueProvider);
     if (queue.isEmpty) return;
     final api = ref.read(quickCaptureApiProvider);
     for (final text in List<String>.from(queue)) {
+      if (!mounted) break;
       try {
         final result = await api.capture(text);
+        if (!mounted) break;
         await ref.read(captureQueueProvider.notifier).dequeue(text);
         switch (result.type) {
           case 'note':
@@ -263,7 +266,7 @@ class _QuickCaptureBarState extends ConsumerState<_QuickCaptureBar> {
         break; // Still offline — stop draining.
       } catch (_) {
         // Server/parse error — remove to avoid infinite retries.
-        await ref.read(captureQueueProvider.notifier).dequeue(text);
+        if (mounted) await ref.read(captureQueueProvider.notifier).dequeue(text);
       }
     }
   }