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)

nixserver/src/index.css

@@ -0,0 +1,22 @@
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
+
+* {
+  margin: 0;
+  padding: 0;
+  box-sizing: border-box;
+}
+
+html {
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', 'Roboto', 'Oxygen',
+    'Ubuntu', 'Cantarell', 'Fira Sans', 'Droid Sans', 'Helvetica Neue',
+    sans-serif;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+
+body {
+  background-color: rgb(3, 7, 30);
+  color: rgb(229, 231, 235);
+}