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/browser.py

@@ -24,6 +24,15 @@ logger = logging.getLogger(__name__)
 router = APIRouter(prefix="/api/browser", tags=["browser"])
 
 
+def _require_folder_management() -> None:
+    """Raise 403 if media folder management is disabled in config."""
+    if not settings.media_folders_management:
+        raise HTTPException(
+            status_code=403,
+            detail="Media folder management is disabled. Set media_folders_management: true in config.yaml to enable.",
+        )
+
+
 async def _broadcast_after_open(controller, label: str, max_wait: float = 2.0) -> None:
     """Poll until media session registers, then broadcast status update.
 
@@ -83,17 +92,22 @@ async def list_folders(_: str = Depends(verify_token)):
     """List all configured media folders.
 
     Returns:
-        Dictionary of folder configurations.
+        Dictionary with folder configurations and management flag.
     """
     folders = {}
     for folder_id, config in settings.media_folders.items():
+        folder_path = Path(config.path)
         folders[folder_id] = {
             "id": folder_id,
             "label": config.label,
             "path": config.path,
             "enabled": config.enabled,
+            "available": folder_path.is_dir(),
         }
-    return folders
+    return {
+        "folders": folders,
+        "management_enabled": settings.media_folders_management,
+    }
 
 
 @router.post("/folders/create")
@@ -112,6 +126,7 @@ async def create_folder(
     Raises:
         HTTPException: If folder already exists or validation fails.
     """
+    _require_folder_management()
     try:
         # Validate folder_id format (alphanumeric and underscore only)
         if not request.folder_id.replace("_", "").isalnum():
@@ -169,6 +184,7 @@ async def update_folder(
     Raises:
         HTTPException: If folder doesn't exist or validation fails.
     """
+    _require_folder_management()
     try:
         # Validate path exists
         path = Path(request.path)
@@ -217,6 +233,7 @@ async def delete_folder(
     Raises:
         HTTPException: If folder doesn't exist.
     """
+    _require_folder_management()
     try:
         config_manager.delete_media_folder(folder_id)