feat(i18n): localize settings + search screens

Extend the en/zh localization to the two screens reached from the home
app bar (the "全都用 + 多语言" ask):
- settings: title, theme-mode segments (System/Light/Dark), seed-color
  description, color-picker + clear-data dialogs.
- search: hint, error, empty/no-results states, Notes/Documents section
  headers, page label.

New ARB keys regenerated; l10n_test now asserts the new keys resolve in
both English and Chinese.

flutter analyze: 0 issues. l10n_test: 3/3.

lib/l10n/app_localizations_en.dart

@@ -53,6 +53,60 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get removeDocument => 'Remove document';
 
+  @override
+  String get ok => 'OK';
+
+  @override
+  String get pickColor => 'Pick a color';
+
+  @override
+  String get clearSettingsTitle => 'Clear all local settings?';
+
+  @override
+  String get clear => 'Clear';
+
+  @override
+  String get settingsReset => 'Settings reset to defaults';
+
+  @override
+  String get themeSystem => 'System';
+
+  @override
+  String get themeLight => 'Light';
+
+  @override
+  String get themeDark => 'Dark';
+
+  @override
+  String get seedColorDesc => 'Seed color for Material 3 theme';
+
+  @override
+  String get searchHint => 'Search notes and documents...';
+
+  @override
+  String searchError(String error) {
+    return 'Search error: $error';
+  }
+
+  @override
+  String noResultsFor(String query) {
+    return 'No results for \"$query\"';
+  }
+
+  @override
+  String get typeToSearch => 'Type to search your notes and documents';
+
+  @override
+  String get sectionNotes => 'Notes';
+
+  @override
+  String get sectionDocuments => 'Documents';
+
+  @override
+  String pageLabel(int page) {
+    return 'Page $page';
+  }
+
   @override
   String get processingPptx => 'Processing PPTX...';