build(web): generate the UI's Tailwind CSS with the pinned standalone CLI (#685)

Replaces the hand-written Tailwind subset in app.css with a real, purged
Tailwind build: scripts/build_css.py runs the pinned, SHA-256-checked
standalone Tailwind CLI (no Node), the generated tailwind.css and
plugin-frame.css are committed, and CI fails when they are stale. The Pi
never builds anything. The login page (#683) now links tailwind.css too,
and the load-order test covers every template that links app.css.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/templates/v3/base.html

@@ -326,6 +326,10 @@
          ../webfonts/ relatively, so both live under v3/vendor/fontawesome/) -->
     <link rel="stylesheet" href="{{ url_for('static', filename='v3/vendor/fontawesome/css/all.min.css') }}">
 
+    <!-- Utility classes, generated by scripts/build_css.py (Tailwind CLI) and
+         committed. app.css comes second so its components and theme overrides
+         win over utilities of equal specificity. -->
+    <link rel="stylesheet" href="{{ url_for('static', filename='v3/tailwind.css') }}">
     <!-- Custom v3 styles -->
     <link rel="stylesheet" href="{{ url_for('static', filename='v3/app.css') }}">
 </head>

web_interface/templates/v3/login.html

@@ -20,6 +20,8 @@
     })();
     </script>
     <link rel="stylesheet" href="{{ url_for('static', filename='v3/vendor/fontawesome/css/all.min.css') }}">
+    <!-- Utilities from scripts/build_css.py; app.css second, as in base.html. -->
+    <link rel="stylesheet" href="{{ url_for('static', filename='v3/tailwind.css') }}">
     <link rel="stylesheet" href="{{ url_for('static', filename='v3/app.css') }}">
     <style>
         .login-wrap { max-width: 24rem; margin: 0 auto; padding: 4rem 1rem; }