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

astro.config.mjs

@@ -1,12 +1,7 @@
 // @ts-check
 import { defineConfig } from 'astro/config';
-import tailwind from '@astrojs/tailwind';
 
 // https://astro.build/config
 export default defineConfig({
   output: 'static',
-  integrations: [tailwind()],
-  build: {
-    inlineStylesheets: 'auto',
-  },
 });