chore: update all dependencies to latest (#37)

- Astro 5.1 -> 5.17
- Starlight 0.31 -> 0.37 (social config migrated to array format)
- Tailwind CSS 3 -> 4 (migrated to Vite plugin + CSS-based config)
- sharp 0.32 -> 0.34
- ESLint 8 -> 9 (migrated to flat config)
- Prettier, prettier-plugin-astro, prettier-plugin-tailwindcss updated
- Removed dotenv (using Astro import.meta.env)
- Removed tailwind.config.mjs (config now in CSS via @theme)
- Updated Badge import to new Starlight components path
- Node.js 18 -> 22
- Switched from npm to bun

eslint.config.js

@@ -0,0 +1,5 @@
+import eslintPluginAstro from 'eslint-plugin-astro'
+
+export default [
+  ...eslintPluginAstro.configs['flat/recommended'],
+]