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/config.py

@@ -124,6 +124,10 @@ class Settings(BaseSettings):
         default_factory=dict,
         description="Media folders available for browsing in the media browser",
     )
+    media_folders_management: bool = Field(
+        default=True,
+        description="Allow adding, editing, and deleting media folders from the Web UI",
+    )
 
     # Thumbnail settings
     thumbnail_size: str = Field(