feat: Convert to pure Astro CSS (no Tailwind)

- Removed Tailwind CSS dependency (19KB → 8.7KB CSS)
- Created native Astro CSS with CSS custom properties
- All utility classes using vanilla CSS
- Scoped component styles with Astro's built-in scoping
- Same green theme and design preserved
- Zero build dependencies for CSS
- Faster builds, smaller bundle
- True 'Astro way' of styling

package.json

@@ -9,14 +9,10 @@
     "astro": "astro"
   },
   "dependencies": {
-    "@astrojs/tailwind": "^6.0.2",
     "@libsql/client": "^0.17.0",
     "astro": "^5.17.1",
     "astro-consent": "^1.0.17",
-    "autoprefixer": "^10.4.27",
     "drizzle-orm": "^0.45.1",
-    "postcss": "^8.5.8",
-    "serve": "^14.2.6",
-    "tailwindcss": "^3.4.1"
+    "serve": "^14.2.6"
   }
 }