feat: migrate SCSS → Tailwind CSS + remove sass dependency

12 SCSS files (1167 lines) replaced by centralized index.css with
Tailwind v4 @theme tokens, @layer components, and utility classes.
Game panel design system (gp-*) preserved as CSS components.
Inline styles in Settings/Login/MilestoneBar converted to Tailwind utilities.
sass removed from dependencies. Build clean, 53 tests pass.

Frontend/src/pages/Cookie.jsx

@@ -1,4 +1,3 @@
-import "../scss/Cookie.scss";
 function Cookie() {
   return (
     <div className="container">