Homepage: let go of the presentations' observers when leaving the page

[ngnijland]

Summary

The site uses Astro's ClientRouter, which swaps pages without a reload. The homepage presentations set up observers that were never disconnected, so each client-side visit back to the homepage added another set, and the old ones kept references to the previous page's stages, videos and autoplay state. Nothing broke visibly: the old autoplays stop themselves once their stage is off the page. But the old page's elements could stay in memory.

Each presentation now lets go on astro:before-swap, just before a navigation replaces the page:

• Autoplay (tabs and gallery, autoplay.ts) disconnects its IntersectionObserver, cancels its frame loop and pauses its videos. It is self-contained, so the presentations don't need to call anything.
• Closer look does the same for its own observer and videos.
• Tabs drop their ResizeObserver and the list layout's media query listener on the same event. This replaces the module-level teardown list from Homepage: give the feature tabs motion of their own #1207 and Homepage: list the feature tabs beside the stage on wide screens #1208, so all three presentations clean up the same way.

Test plan

• npm run lint, format:check, typecheck, build, knip in docs/; npm test (93 pass)
• Headless Chrome with wrapped observers, navigating client-side from /home-next/ to /pricing/ and back:
  • First load: 6 stage IntersectionObservers and 4 tabs ResizeObservers.
  • After leaving: all 6 and all 4 disconnected (before: 0).
  • Back again: fresh ones are created, Teams autoplay still advances, and stage swipes still work.

🤖 Generated with Claude Code