Redesign marketing site with Astro (#187)

* marketing site

* redesign 1

* more

* Redesign marketing site: port to Astro, minimal dark homepage

Replace the React/Vite marketing site with a single static Astro page.
Dark theme matching the app aesthetic, featuring the production glossy
T3 icon, app screenshot, and a clean minimal layout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Fix GitHub link to pingdotgg/t3code

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>