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/lib/api.ts

@@ -370,6 +370,8 @@ export const api = {
     req(`/api/playlists/watch-later/${videoId}`, { method: "DELETE" }),
   syncYoutubePlaylists: (): Promise<{ synced: number; reason?: string }> =>
     req("/api/playlists/sync-youtube", { method: "POST" }),
+  revertPlaylist: (id: number): Promise<{ items: number }> =>
+    req(`/api/playlists/${id}/revert-youtube`, { method: "POST" }),
   playlistPushPlan: (id: number): Promise<PushPlan> =>
     req(`/api/playlists/${id}/push-plan`),
   pushPlaylist: (id: number): Promise<PushResult> =>