feat(playlists): say which lists YouTube's API cannot give us

The module shows fewer lists than youtube.com, and without a word about it that read as a Siftlode
bug (the owner's ask, 2026-09-03). Owner's pick from the S7 mockups (2026-09-19): one quiet line at
the foot of the rail, "Missing a list? Why?", whose Why? opens three facts — YouTube's own Watch
later and History are closed to its API (the Watch later list here is Siftlode's own), only lists
you own sync, and an account with no YouTube channel has no playlists at all.

frontend/src/components/PlaylistsRail.tsx

@@ -22,6 +22,7 @@ import { usePlaylists, usePlaylistsActions } from "./playlistsContext";
 import { useMe } from "../lib/useMe";
 import { playlistName } from "../lib/playlistName";
 import { usePlaylistActions } from "./usePlaylistActions";
+import PlaylistLimitsHint from "./PlaylistLimitsHint";
 
 type PlSort = {
   key: "custom" | "name" | "count" | "duration" | "recent";
@@ -382,6 +383,7 @@ export default function PlaylistsRail({
             )}
           </div>
           <ContextMenu menu={railMenu} ariaLabel={t("playlists.railMenu")} items={railItems} />
+          <PlaylistLimitsHint />
         </>
       ),
     },