feat: initial website commit + installation guide page

- Full nixpanel-website monorepo: web (React/Vite) + api (Fastify)
- Add /install page with one-liner, manual steps, requirements,
  software table, firewall ports, service management, and update/uninstall
- Add Install link to header nav and footer
- Add 'Install guide' CTA button to home page hero
- Add 'outline' variant to Button component
- Add .gitignore and .gitattributes

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

web/src/index.css

@@ -0,0 +1,9 @@
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
+
+@layer base {
+  body {
+    @apply bg-gray-950 text-gray-100 font-sans antialiased;
+  }
+}