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/sx/templates/pages.sx

@@ -10,7 +10,7 @@
           "body{margin:0;min-height:100vh;display:flex;align-items:center;"
           "justify-content:center;font-family:system-ui,sans-serif;"
           "background:#fafaf9;color:#1c1917}")
-        (script :src "https://cdn.tailwindcss.com")
+        (link :rel "stylesheet" :type "text/css" :href (str asset-url "/styles/tw.css"))
         (link :rel "stylesheet" :href (str asset-url "/fontawesome/css/all.min.css")))
       (body :class "bg-stone-50 text-stone-900"
         children))))