feat: add NixServer and NixClient React UIs, wire into installer

- nixserver/: React 18 + TypeScript + Vite + Tailwind
  - Login page + placeholder admin dashboard with card grid
  - Blue accent color (#3b82f6)
  - Full Tailwind + PostCSS + TypeScript setup
- nixclient/: identical structure, user-facing panel
  - Green accent color (#10b981)
  - Same build tooling as NixServer
- install.sh: new 'Building NixServer & NixClient UIs' step
  - npm install + npm run build for each
  - Outputs to /opt/nixpanel/nixserver/dist and /opt/nixpanel/nixclient/dist
  - Both panels immediately accessible via Apache on 2087 and 2083
  - Skips gracefully if source not present (early dev)

nixclient/src/main.tsx

@@ -0,0 +1,10 @@
+import React from 'react'
+import ReactDOM from 'react-dom/client'
+import App from './App'
+import './index.css'
+
+ReactDOM.createRoot(document.getElementById('root')!).render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>,
+)