Add search, offline queue, app icon, and UI polish

- Collapsible search in notes and tasks: magnifying glass in AppBar
  expands to a text field inline; close button resets the filter
- Offline capture queue: failed quick-captures (NetworkException) are
  persisted to SharedPreferences and retried automatically on next
  successful submit or app start; badge shows pending count
- App icon: book-with-sparkle logo from FabledAssistant SVG rendered
  at all Android densities with adaptive icon (indigo #6366f1 bg)
- Dark mode subtitle fix: use colorScheme.onSurfaceVariant for note
  preview and conversation timestamp text
- Remove swipe-to-delete (accidental deletions); long-press remains
- Chat: SSE streaming reliability, polling fallback, title patching
- Settings: theme toggle (system/light/dark) persisted to prefs
- Notes: delete button in edit screen; body preview in list
- Tasks: fix delete dialog context; description search support

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

lib/screens/settings/settings_screen.dart

@@ -12,6 +12,7 @@ class SettingsScreen extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final serverUrl = ref.watch(serverUrlProvider);
+    final themeMode = ref.watch(themeModeProvider);
 
     return Scaffold(
       appBar: AppBar(title: const Text('Settings')),
@@ -24,6 +25,33 @@ class SettingsScreen extends ConsumerWidget {
             onTap: () => context.go(Routes.setup),
           ),
           const Divider(),
+          ListTile(
+            title: const Text('Appearance'),
+            leading: const Icon(Icons.brightness_6),
+            trailing: SegmentedButton<ThemeMode>(
+              segments: const [
+                ButtonSegment(
+                  value: ThemeMode.system,
+                  icon: Icon(Icons.brightness_auto),
+                  tooltip: 'System',
+                ),
+                ButtonSegment(
+                  value: ThemeMode.light,
+                  icon: Icon(Icons.light_mode),
+                  tooltip: 'Light',
+                ),
+                ButtonSegment(
+                  value: ThemeMode.dark,
+                  icon: Icon(Icons.dark_mode),
+                  tooltip: 'Dark',
+                ),
+              ],
+              selected: {themeMode},
+              onSelectionChanged: (modes) =>
+                  ref.read(themeModeProvider.notifier).setMode(modes.first),
+            ),
+          ),
+          const Divider(),
           ListTile(
             title: const Text('Sign Out'),
             leading: const Icon(Icons.logout),