feat(settings): add search functionality to settings modal (P2)

Implement Phase 2 search bar feature for settings modal:

- Add search input to settings modal header with icon and clear button
- Implement real-time filtering with 150ms debounce for performance
- Add visual highlighting for matched search terms using accent color
- Implement empty search results state with user-friendly message
- Add keyboard shortcuts (Escape to clear search)
- Auto-expand sections containing matching content during search
- Fix header layout to prevent overlap with close button
- Update progress tracker documenting P2 completion
- Add translation keys for search feature (placeholder, clear, no results)
- Sync translations across all language files

Files changed:
- templates/components/modals/settings_modal.html
- static/css/components/modal/settings-modal.css
- static/js/managers/SettingsManager.js
- locales/*.json (10 language files)
- docs/ui-ux-optimization/progress-tracker.md

locales/zh-CN.json

@@ -275,6 +275,11 @@
             "download": "[TODO: Translate] Download",
             "advanced": "[TODO: Translate] Advanced"
         },
+        "search": {
+            "placeholder": "[TODO: Translate] Search settings...",
+            "clear": "[TODO: Translate] Clear search",
+            "noResults": "[TODO: Translate] No settings found matching \"{query}\""
+        },
         "storage": {
             "locationLabel": "便携模式",
             "locationHelp": "开启可将 settings.json 保存在仓库中；关闭则保存在用户配置目录。"