style: fix biome check errors (import order + formatting)

npx biome check reported 14 errors on app/public/assets/css/,
app/public/assets/js/app/ and scripts/:
  - assist/source/organizeImports: scripts/css-content-sources.mjs,
    scripts/css-unused-report.mjs
  - format: 9 CSS files + 3 JS files

Fixed via biome check --write. Verified: npx biome check clean,
npm run build succeeds, PHPUnit 316 tests / 628 assertions pass.

app/public/assets/css/components/buttons.css

@@ -205,7 +205,7 @@
 
 /* Small arrow below the bubble pointing at the button */
 .btn[data-tip]::before {
-	content: '';
+	content: "";
 	position: absolute;
 	left: 50%;
 	bottom: calc(100% + 4px);