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/static/locales/en.json

@@ -173,7 +173,27 @@
   "browser.play_all_error": "Failed to play folder",
   "browser.error_loading": "Error loading directory",
   "browser.error_loading_folders": "Failed to load media folders",
-  "browser.manage_folders_hint": "Folder management coming soon! For now, edit config.yaml to add media folders.",
+  "browser.manage_folders_hint": "Folder management is disabled. Set media_folders_management: true in config.yaml to enable.",
+  "browser.unavailable": "Unavailable",
+  "browser.folder_available": "Available",
+  "browser.folder_unavailable": "Unavailable (path not reachable)",
+  "browser.folder_disabled": "disabled",
+  "browser.folder_edit": "Edit folder",
+  "browser.folder_delete": "Delete folder",
+  "browser.folder_created": "Media folder created successfully",
+  "browser.folder_updated": "Media folder updated successfully",
+  "browser.folder_deleted": "Media folder deleted successfully",
+  "browser.folder_save_error": "Failed to save media folder",
+  "browser.folder_delete_error": "Failed to delete media folder",
+  "browser.folder_confirm_delete": "Are you sure you want to delete the folder \"{name}\"?",
+  "browser.folders_description": "Media folders available for browsing. Folders on network shares show availability status.",
+  "browser.folders_empty": "No media folders configured. Click \"+\" to add one.",
+  "browser.folders_table.id": "ID",
+  "browser.folders_table.label": "Label",
+  "browser.folders_table.path": "Path",
+  "browser.folders_table.status": "Status",
+  "browser.folders_table.actions": "Actions",
+  "settings.section.media_folders": "Media Folders",
   "browser.folder_dialog.title_add": "Add Media Folder",
   "browser.folder_dialog.title_edit": "Edit Media Folder",
   "browser.folder_dialog.folder_id": "Folder ID *",