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.dart

@@ -188,6 +188,102 @@ abstract class AppLocalizations {
   /// **'Remove document'**
   String get removeDocument;
 
+  /// No description provided for @ok.
+  ///
+  /// In en, this message translates to:
+  /// **'OK'**
+  String get ok;
+
+  /// No description provided for @pickColor.
+  ///
+  /// In en, this message translates to:
+  /// **'Pick a color'**
+  String get pickColor;
+
+  /// No description provided for @clearSettingsTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Clear all local settings?'**
+  String get clearSettingsTitle;
+
+  /// No description provided for @clear.
+  ///
+  /// In en, this message translates to:
+  /// **'Clear'**
+  String get clear;
+
+  /// No description provided for @settingsReset.
+  ///
+  /// In en, this message translates to:
+  /// **'Settings reset to defaults'**
+  String get settingsReset;
+
+  /// No description provided for @themeSystem.
+  ///
+  /// In en, this message translates to:
+  /// **'System'**
+  String get themeSystem;
+
+  /// No description provided for @themeLight.
+  ///
+  /// In en, this message translates to:
+  /// **'Light'**
+  String get themeLight;
+
+  /// No description provided for @themeDark.
+  ///
+  /// In en, this message translates to:
+  /// **'Dark'**
+  String get themeDark;
+
+  /// No description provided for @seedColorDesc.
+  ///
+  /// In en, this message translates to:
+  /// **'Seed color for Material 3 theme'**
+  String get seedColorDesc;
+
+  /// No description provided for @searchHint.
+  ///
+  /// In en, this message translates to:
+  /// **'Search notes and documents...'**
+  String get searchHint;
+
+  /// No description provided for @searchError.
+  ///
+  /// In en, this message translates to:
+  /// **'Search error: {error}'**
+  String searchError(String error);
+
+  /// No description provided for @noResultsFor.
+  ///
+  /// In en, this message translates to:
+  /// **'No results for \"{query}\"'**
+  String noResultsFor(String query);
+
+  /// No description provided for @typeToSearch.
+  ///
+  /// In en, this message translates to:
+  /// **'Type to search your notes and documents'**
+  String get typeToSearch;
+
+  /// No description provided for @sectionNotes.
+  ///
+  /// In en, this message translates to:
+  /// **'Notes'**
+  String get sectionNotes;
+
+  /// No description provided for @sectionDocuments.
+  ///
+  /// In en, this message translates to:
+  /// **'Documents'**
+  String get sectionDocuments;
+
+  /// No description provided for @pageLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Page {page}'**
+  String pageLabel(int page);
+
   /// No description provided for @processingPptx.
   ///
   /// In en, this message translates to: