Tabbed UI, browse caching, and bottom mini player

- Convert stacked sections to tabbed interface (Player, Browser, Actions, Scripts, Callbacks) with localStorage persistence
- Add in-memory directory listing cache (5-min TTL) with nocache bypass for refresh
- Defer stat()/duration calls to paginated items only for faster browse
- Move mini player from top to bottom with footer padding fix
- Always show scrollbar to prevent layout shift between tabs
- Add tab localization keys (en/ru)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

media_server/static/locales/en.json

@@ -109,6 +109,11 @@
   "callbacks.msg.list_failed": "Failed to load callbacks",
   "callbacks.confirm.delete": "Are you sure you want to delete the callback \"{name}\"?",
   "callbacks.confirm.unsaved": "You have unsaved changes. Are you sure you want to discard them?",
+  "tab.player": "Player",
+  "tab.browser": "Browser",
+  "tab.quick_actions": "Actions",
+  "tab.scripts": "Scripts",
+  "tab.callbacks": "Callbacks",
   "browser.title": "Media Browser",
   "browser.home": "Home",
   "browser.manage_folders": "Manage Folders",