feat(playlists): reset a playlist to its YouTube state (discard local edits)

The in-session undo/redo is lost when switching playlists, leaving no way to undo
local edits to a YouTube-linked list afterwards. Add a per-playlist 'Reset to YouTube'
action (shown when a linked playlist is dirty) that force-repulls that single playlist
from YouTube, replacing its items/order/name and clearing dirty — even though the bulk
read-sync skips dirty playlists. Backend: repull_playlist() + POST /api/playlists/
{id}/revert-youtube (read-scope gated). Confirm dialog (destructive). Trilingual.

frontend/src/i18n/locales/en/playlists.json

@@ -20,6 +20,12 @@
   "exportToYoutube": "Export to YouTube",
   "pushToYoutube": "Sync to YouTube",
   "unsynced": "Unsynced changes",
+  "revert": "Reset to YouTube",
+  "revertTitle": "Discard local changes?",
+  "revertMsg": "This reloads the playlist from YouTube and discards your unsynced local changes (order, additions, removals, rename). Continue?",
+  "revertConfirm": "Discard & reload",
+  "revertDone": "Reloaded from YouTube ✓",
+  "revertFailed": "Couldn't reload from YouTube.",
   "pushTitle": "Sync to YouTube",
   "pushConfirm": "Sync",
   "pushPlanCreate": "Create a new YouTube playlist with {{count}} videos? (~{{units}} quota units; {{left}} left today.)",