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/static/v3/js/widgets/README.md

@@ -354,6 +354,12 @@ widget on one of those is ignored.
   selector: allow only `[A-Za-z0-9_-]`. `BaseWidget` has `sanitizeId()`.
 - Associate labels with inputs and keep the widget usable from the keyboard.
 - Debounce events that fire on every keystroke.
+- Style with the Tailwind utility classes the core UI already uses (`px-3`,
+  `border-gray-300`, `rounded`, `text-sm`, ...). `static/v3/tailwind.css` is
+  generated from the core templates and JS, so it only contains classes core
+  uses, plus everything the hand-written stylesheet defined before the build.
+  A rarer class a plugin widget needs belongs in a `<style>` the widget
+  injects, since core can't scan plugin repos.
 
 ## Troubleshooting