Improve code block UX with share and open in playground

The copy button script in src/layouts/BaseLayout.astro:58 works, but it's the only thing we have for a site that writes a lot about code.

Upgrade:
- keep the copy button but add a toast ("Copied") instead of just swapping the icon
- add filename tab when markdown has a title, and show language label
- add "Open in StackBlitz" for runnable examples — reuse the Cookie Store and Durable Streams demos in src/components/blog/* as patterns
- track most-copied blocks with a simple event to PostHog/Plausible so I know what's useful
- make sure it still works after astro:page-load transitions and with keyboard users

No need to rewrite syntax highlighting. Shiki config in astro.config.mjs:62 stays as is. Just the wrapper around pre.astro-code.