feat(start): tutorial-rail implementation for the playlist fetch

Source half of b672023 (which captured only the removed bundled
thumbnails — the add aborted on the already-staged deletions): the
videos module (playlist scan, oEmbed titles, config-dir thumbnail
cache), the boot fetch on its own thread with the loading state, the
overlay rail + Videos tab on the Start page, and the earlier tabbed-
layout handover.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main.rs

@@ -13,6 +13,7 @@ mod entities;
 mod io;
 mod modules;
 mod patreon;
+mod videos;
 mod plugin;
 mod scene;
 mod snap;