fix: add astro.config.mjs with @tailwindcss/vite plugin

Tailwind 4 was producing only theme variables (oklch colors)
without any utility classes because Astro had no astro.config.mjs
wiring the @tailwindcss/vite plugin. Result: site rendered
unstyled despite all components and HTML being correct.

Verified locally after fix:
- CSS bundle grew from 21 KB (theme only) to 36 KB (theme + utilities)
- 23 utility-class definitions present (bg-neutral-900, text-white,
  min-h-screen, backdrop-blur, font-bold, flex etc)

Also adds @source directive in global.css to ensure Tailwind
scans .astro files for class usage.

astro.config.mjs

@@ -0,0 +1,8 @@
+import { defineConfig } from 'astro/config';
+import tailwindcss from '@tailwindcss/vite';
+
+export default defineConfig({
+  vite: {
+    plugins: [tailwindcss()],
+  },
+});

src/styles/global.css

@@ -1 +1,4 @@
-@import "tailwindcss";
+@import "tailwindcss";
+
+@source "../**/*.{astro,html,js,jsx,ts,tsx,md,mdx,svelte,vue}";
+@source "../../public/**/*.html";