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/vite.config.ts

@@ -0,0 +1,15 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  plugins: [react()],
+  build: {
+    outDir: 'dist',
+    sourcemap: false,
+    minify: 'terser',
+  },
+  server: {
+    port: 3001,
+    strictPort: true,
+  },
+})