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/screens/search_screen.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../l10n/app_localizations.dart';
 import '../models/note.dart';
 import '../providers/search_provider.dart';
 import 'note_editor_screen.dart';
@@ -42,16 +43,17 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
   @override
   Widget build(BuildContext context) {
     final results = ref.watch(searchResultsProvider);
+    final l = AppLocalizations.of(context);
 
     return Scaffold(
       appBar: AppBar(
         title: TextField(
           controller: _controller,
           autofocus: true,
-          decoration: const InputDecoration(
-            hintText: 'Search notes and documents...',
+          decoration: InputDecoration(
+            hintText: l.searchHint,
             border: InputBorder.none,
-            hintStyle: TextStyle(color: Colors.grey),
+            hintStyle: const TextStyle(color: Colors.grey),
           ),
           onChanged: _onQueryChanged,
           onSubmitted: (value) {
@@ -72,14 +74,14 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
       ),
       body: results.when(
         loading: () => const Center(child: CircularProgressIndicator()),
-        error: (e, _) => Center(child: Text('Search error: $e')),
+        error: (e, _) => Center(child: Text(l.searchError('$e'))),
         data: (hits) {
           final query = ref.watch(searchQueryProvider);
           if (query.isEmpty) {
-            return const Center(
+            return Center(
               child: Text(
-                'Type to search your notes and documents',
-                style: TextStyle(color: Colors.grey),
+                l.typeToSearch,
+                style: const TextStyle(color: Colors.grey),
               ),
             );
           }
@@ -95,7 +97,7 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
                   ),
                   const SizedBox(height: 16),
                   Text(
-                    'No results for "$query"',
+                    l.noResultsFor(query),
                     style: Theme.of(context).textTheme.bodyLarge?.copyWith(
                       color: Theme.of(context).colorScheme.onSurfaceVariant,
                     ),
@@ -114,7 +116,7 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
                 Padding(
                   padding: const EdgeInsets.fromLTRB(16, 12, 16, 4),
                   child: Text(
-                    'Notes',
+                    l.sectionNotes,
                     style: Theme.of(context).textTheme.titleSmall?.copyWith(
                       fontWeight: FontWeight.bold,
                       color: Theme.of(context).colorScheme.primary,
@@ -129,7 +131,7 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
                 Padding(
                   padding: const EdgeInsets.fromLTRB(16, 16, 16, 4),
                   child: Text(
-                    'Documents',
+                    l.sectionDocuments,
                     style: Theme.of(context).textTheme.titleSmall?.copyWith(
                       fontWeight: FontWeight.bold,
                       color: Theme.of(context).colorScheme.primary,
@@ -209,7 +211,7 @@ class _DocumentSearchResultTile extends StatelessWidget {
         crossAxisAlignment: CrossAxisAlignment.start,
         children: [
           Text(
-            'Page ${pageNumber + 1}',
+            AppLocalizations.of(context).pageLabel(pageNumber + 1),
             style: Theme.of(context).textTheme.bodySmall,
           ),
           if (snippet.isNotEmpty)