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/providers/settings_provider.dart

@@ -1,7 +1,9 @@
+import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 const _kServerUrl = 'server_url';
+const _kThemeMode = 'theme_mode';
 
 final sharedPreferencesProvider = Provider<SharedPreferences>((ref) {
   throw UnimplementedError('Override in ProviderScope');
@@ -11,6 +13,35 @@ final cookiesPathProvider = Provider<String>((ref) {
   throw UnimplementedError('Override in ProviderScope');
 });
 
+final themeModeProvider =
+    StateNotifierProvider<ThemeModeNotifier, ThemeMode>((ref) {
+  final prefs = ref.watch(sharedPreferencesProvider);
+  return ThemeModeNotifier(prefs);
+});
+
+class ThemeModeNotifier extends StateNotifier<ThemeMode> {
+  final SharedPreferences _prefs;
+
+  ThemeModeNotifier(this._prefs)
+      : super(_fromString(_prefs.getString(_kThemeMode)));
+
+  static ThemeMode _fromString(String? value) => switch (value) {
+        'light' => ThemeMode.light,
+        'dark' => ThemeMode.dark,
+        _ => ThemeMode.system,
+      };
+
+  Future<void> setMode(ThemeMode mode) async {
+    final value = switch (mode) {
+      ThemeMode.light => 'light',
+      ThemeMode.dark => 'dark',
+      _ => 'system',
+    };
+    await _prefs.setString(_kThemeMode, value);
+    state = mode;
+  }
+}
+
 final serverUrlProvider = StateNotifierProvider<ServerUrlNotifier, String?>((ref) {
   final prefs = ref.watch(sharedPreferencesProvider);
   return ServerUrlNotifier(prefs);