Add browser search/filter for media items

- Search bar appears when browsing inside a folder
- Client-side filtering with 200ms debounce
- Clear button and search icon
- Hides at root level, resets on navigation
- Localized placeholder (en/ru)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

media_server/static/locales/en.json

@@ -124,6 +124,7 @@
   "browser.view_grid": "Grid view",
   "browser.view_compact": "Compact view",
   "browser.view_list": "List view",
+  "browser.search": "Search...",
   "browser.items_per_page": "Items per page:",
   "browser.page": "Page",
   "browser.previous": "Previous",