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

config.example.yaml

@@ -56,6 +56,11 @@ scripts:
     timeout: 10
     shell: true
 
+# Media folder management from Web UI (default: true)
+# When enabled, media folders can be added, edited, and deleted from the Settings tab.
+# Set to false to disable folder management from the UI.
+# media_folders_management: false
+
 # Callback scripts (executed after media actions)
 # All callbacks are optional - if not defined, the action runs without callback
 callbacks: