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/components/buttons/PrimaryButton.jsx

@@ -1,5 +1,4 @@
 import PropTypes from "prop-types";
-import "../../scss/components/buttons.scss";
 import { Link } from "react-router-dom";
 
 export default function PrimaryButton({ btnText, btnLink }) {