Rebuild Tailwind CSS for presign upload spinner

The new spinner / busy-state markup added to _upload_form.html in
18faa17 introduced fresh Tailwind classes that weren't reflected in
the bundled flask_s3_viewer/blueprints/static/css/app.css, so the CI
"Verify Tailwind build is up-to-date" step was flagging the working
tree as out of sync. Regenerate the minified bundle with the standard
frontend/npm run build pipeline.

Author: hidekuma