Replace Tailwind Play CDN with a prebuilt, purged stylesheet

The runtime CDN (cdn.tailwindcss.com) is not production-grade: FOUC, no
purging, and an external request that is slow/blocked from some networks.

- Add Tailwind v3 build (package.json `npm run build`) with two scoped configs:
  public (accent + zinc) -> wwwroot/css/tailwind.css, and admin (dark base/
  electric/violet/emerald, separate to avoid the emerald flat-vs-scale clash)
  -> wwwroot/css/tailwind-admin.css. Both minified + content-purged.
- Layouts now link the built CSS instead of the CDN script; built artifacts
  are committed so Docker/CI need no Node step. node_modules stays ignored.
- Verified: utilities (incl. arbitrary values like aspect-[16/9], grid-cols-
  [8rem_1fr]) resolve; public + admin render; no console errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tailwind.admin.config.js

@@ -0,0 +1,26 @@
+/** @type {import('tailwindcss').Config} */
+// Admin (dark) theme. Separate build so its flat custom colors do not collide
+// with the public site's use of built-in color scales (e.g. emerald-600).
+module.exports = {
+  content: [
+    './Pages/Admin/**/*.cshtml',
+    './Pages/Shared/_AdminLayout.cshtml',
+  ],
+  theme: {
+    extend: {
+      colors: {
+        base: { DEFAULT: '#020510', 800: '#050a1a' },
+        electric: '#38bdf8',
+        violet: '#818cf8',
+        magenta: '#e879f9',
+        emerald: '#34d399',
+      },
+      fontFamily: {
+        display: ['Syne', 'system-ui', 'sans-serif'],
+        sans: ['Syne', 'system-ui', 'sans-serif'],
+        mono: ['SpaceMono', 'monospace'],
+      },
+    },
+  },
+  plugins: [],
+};