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/casts-src/casts.mjs

@@ -45,6 +45,7 @@ export const casts = [
     width: 90,
     height: 26,
     typeSpeed: '45ms',
+    holdEnd: 2.5, // landing cast loops — pause on the final frame before restart
     steps: [
       { wait: 1100 },
       { type: 'create table authors with pk', after: 1000 },