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>

docs/WEB_INTERFACE_GUIDE.md

@@ -482,7 +482,9 @@ The web interface uses modern web technologies:
 
 - **Backend:** Flask with Blueprint-based modular design
 - **Frontend:** HTMX for dynamic content, Alpine.js for reactive components
-- **Styling:** Tailwind CSS for responsive design
+- **Styling:** Tailwind CSS utilities, generated at development time and
+  committed (the Pi never builds CSS; see
+  [`web_interface/README.md`](../web_interface/README.md#styling-tailwind-css))
 - **Real-Time:** Server-Sent Events (SSE) for live updates
 
 ### File Locations