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/Achievements.jsx

@@ -1,6 +1,5 @@
 import { useGameStore } from "../store/useGameStore";
 import { ACHIEVEMENTS } from "../data/achievements";
-import "../scss/achievements.scss";
 
 function Achievements() {
   const state = useGameStore((s) => s.state);