Improve client-side transitions and stabilize theme toggle

[markgaze]

Summary

• Enable Astro ClientRouter transitions across the site and add shared transition names for post cards and post pages so hero imagery/surface transitions animate smoothly.
• Fix theme persistence during client-side navigation by reapplying theme state on transition lifecycle events and preventing duplicate theme event listener bindings.
• Add a Playwright regression test that verifies theme toggling still works after navigating to a post via client routing.

Validation

• pnpm lint
• pnpm build
• pnpm test

[cloudflare-workers-and-pages]

Deploying blog with    Cloudflare Pages

Latest commit:	cfcf8b3
Status:	✅  Deploy successful!
Preview URL:	https://33d0b797.blog-6kn.pages.dev
Branch Preview URL:	https://client-router.blog-6kn.pages.dev

View logs