fix(guest): add playCollection permission so guests can Play All playlist/album (#103)

- New guest flag playCollection (default OFF), gates play-playlist/play-album
- Keeps playNow's non-destructive semantics intact (Play All clears the queue)
- Admin-toggleable in Settings → 游客模式; default-off, backward-compatible
- Frontend: gate the 播放全部 button on the flag + surface 403 as a toast
  instead of failing silently (the silent-failure half of the issue)

src/data/permissions.ts

@@ -29,6 +29,8 @@ export interface GuestPermissions {
   transport: boolean;
   removeClear: boolean;
   playMode: boolean;
+  /** Load + play an entire playlist/album (clears the queue). Issue #103. */
+  playCollection: boolean;
 }
 
 export const GUEST_PERMISSION_FLAGS = [
@@ -39,6 +41,7 @@ export const GUEST_PERMISSION_FLAGS = [
   "transport",
   "removeClear",
   "playMode",
+  "playCollection",
 ] as const;
 export type GuestFlag = (typeof GUEST_PERMISSION_FLAGS)[number];