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/package.json

@@ -17,7 +17,6 @@
     "react-helmet": "^6.1.0",
     "react-lottie-player": "^1.5.5",
     "react-router-dom": "^6.19.0",
-    "sass": "^1.69.5",
     "tailwindcss": "^4.2.2",
     "zustand": "^5.0.12"
   },