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>

Pages/Shared/_Layout.cshtml

@@ -25,22 +25,8 @@
     @@font-face { font-family:'Vazirmatn'; src:url('/fonts/Vazirmatn-Arabic.woff2') format('woff2'); font-display:swap; }
   </style>
 
-  <!-- Tailwind Play CDN - minimal config: one accent, neutral zinc scale is built in -->
-  <script src="https://cdn.tailwindcss.com"></script>
-  <script>
-    tailwind.config = {
-      theme: {
-        extend: {
-          colors: { accent: '#2563eb', accentink: '#1d4ed8' },
-          fontFamily: {
-            display: ['Syne', 'system-ui', 'sans-serif'],
-            fa:      ['Vazirmatn', 'system-ui', 'sans-serif'],
-          },
-        }
-      }
-    }
-  </script>
-
+  <!-- Tailwind: prebuilt + purged stylesheet (`npm run build`). No runtime CDN. -->
+  <link rel="stylesheet" href="/css/tailwind.css" />
   <link rel="stylesheet" href="/css/site.css" />
   <link rel="icon" href="/logo-mark.svg" type="image/svg+xml" />
 </head>