feat: add media folder management from WebUI

- Add media_folders_management config flag (enabled by default)
- Guard folder CRUD endpoints with 403 when management disabled
- Wire up frontend folder add/edit/delete in Settings tab
- Add per-folder availability check (for network shares)
- Show unavailable badge on offline folders in browser view
- Expose management flag via /api/health endpoint
- Add EN/RU locale keys for folder management UI

media_server/routes/health.py

@@ -7,6 +7,7 @@ from fastapi import APIRouter, Request
 
 from .. import __version__
 from ..auth import auth_enabled
+from ..config import settings
 
 router = APIRouter(prefix="/api", tags=["health"])
 
@@ -23,6 +24,7 @@ async def health_check(request: Request) -> dict[str, Any]:
         "platform": platform.system(),
         "version": __version__,
         "auth_required": auth_enabled(),
+        "media_folders_management": settings.media_folders_management,
     }
 
     # Include cached update info if available