Replace Tailwind CDN with pre-built CSS via standalone CLI

- Add shared/static/styles/tailwind.css as Tailwind v4 input with
  explicit @source paths for all service templates and safelisted
  dynamic classes (bg-{colour}-{shade}, text-{size})
- Build to shared/static/styles/tw.css (93KB minified)
- Replace <script src="cdn.tailwindcss.com"> with <link> to tw.css
  in sx page shell, Jinja _head.html, and ~base-shell component
- Add build-tw.sh convenience script

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/browser/templates/_types/root/_head.html

@@ -6,7 +6,7 @@
     <link rel="stylesheet" type="text/css" href="{{asset_url('styles/blog-content.css')}}">
     
     <meta name="csrf-token" content="{{ csrf_token() }}">
-    <script src="https://cdn.tailwindcss.com"></script>
+    <link rel="stylesheet" type="text/css" href="{{asset_url('styles/tw.css')}}">
     <link rel="stylesheet" href="{{asset_url('fontawesome/css/all.min.css')}}">
     <link rel="stylesheet" href="{{asset_url('fontawesome/css/v4-shims.min.css')}}">
     <link href="https://unpkg.com/prismjs/themes/prism.css" rel="stylesheet" />