📝 docs: add Astro Starlight public documentation site

Add comprehensive documentation site using Astro Starlight with:
- Getting Started guides (introduction, quick-start, project structure)
- Development docs (local dev, env vars, docker, migrations, testing)
- Architecture docs (overview, auth, backend, web, mobile, storage, search)
- Guidelines (code style, error handling, database, design/UX)
- Deployment docs (overview, Cloudflare Pages, Mac Mini, self-hosting)
- Project pages (overview, chat)
- API reference structure

Features:
- Dark mode support
- Full-text search (Pagefind)
- Tailwind CSS styling
- Cloudflare Pages deployment ready
- Edit on GitHub links

apps/docs/src/assets/logo-dark.svg

@@ -0,0 +1,5 @@
+<svg width="32" height="32" viewBox="0 0 32 32" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <rect width="32" height="32" rx="8" fill="#0ea5e9"/>
+  <path d="M8 12L16 8L24 12V20L16 24L8 20V12Z" stroke="white" stroke-width="2" fill="none"/>
+  <circle cx="16" cy="16" r="3" fill="white"/>
+</svg>

apps/docs/src/assets/logo-light.svg

@@ -0,0 +1,5 @@
+<svg width="32" height="32" viewBox="0 0 32 32" fill="none" xmlns="http://www.w3.org/2000/svg">
+  <rect width="32" height="32" rx="8" fill="#0ea5e9"/>
+  <path d="M8 12L16 8L24 12V20L16 24L8 20V12Z" stroke="white" stroke-width="2" fill="none"/>
+  <circle cx="16" cy="16" r="3" fill="white"/>
+</svg>