ref(frontend): Replace Bootstrap with Tailwind CSS

Swap the Bootstrap CDN for a self-hosted Tailwind CSS build, mirroring
the myice setup: standalone Tailwind CLI v3.4.17, a one-off build-css.sh
script, and a gitignored generated style.css.

- Add tailwind.config.js, style-input.css, build-css.sh at repo root
- Rewrite templates/index.html: drop Bootstrap link/script, link
  /style.css, convert all classes to Tailwind utilities; showAlert now
  uses a class lookup so the JIT scanner detects dynamic classes
- Add /style.css FileResponse route in main.py; drop cdn.jsdelivr.net
  from script-src/style-src in the CSP
- Flip test assertion to assert cdn.jsdelivr.net is absent from CSP

The generated style.css and tailwindcss binary are gitignored; run
./build-css.sh to regenerate (or --watch for dev).

.gitignore

@@ -131,4 +131,8 @@ dmypy.json
 .idea/
 
 # Additional files from existing .gitignore
-.envrc
+.envrc
+
+# Tailwind CSS generated artifacts
+style.css
+tailwindcss