feat(website): refine casts — trim shell, autoplay+loop landing, cap size

Address cast review feedback:

- Trim every cast to the in-app region (generate.mjs): the recording now
  starts with the app already running and ends on the last in-app frame —
  drops the `$ rdbms-playground` launch and the return-to-shell frame (the
  latter was the stray cursor-under-$ artifact). Opt out per cast with
  `keepShell: true` for demos that document the CLI launch.
- Landing quickstart cast: autoPlay + loop, with a 2.5s hold on the final
  frame so it pauses before restarting.
- Cap the demo at max-width 46rem and centre it, so the player (fit:'width')
  no longer scales its font up to the full splash column.

Casts re-recorded via `pnpm casts`. Build clean (25 pages).

Tab-keypress visibility deferred to an in-app overlay primitive (filed as
issue #22 — also serves the planned guided-lesson system); the cast notes
Tab in its caption for now.

website/src/content/docs/index.mdx

@@ -22,7 +22,7 @@ primary and foreign keys, relationships, indexes, queries, and query
 plans. It is built for learning — from your first table to writing raw
 SQL.
 
-<Demo src="/casts/quickstart.cast" title="From a fresh table to your first query, in simple mode." />
+<Demo src="/casts/quickstart.cast" title="From a fresh table to your first query, in simple mode." autoplay loop />
 
 <CardGrid>
 	<Card title="Two ways to work" icon="seti:db">