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/navbar.jsx

@@ -1,9 +1,6 @@
 import { NavLink as Link } from "react-router-dom";
 import PropTypes from "prop-types";
 
-import "../scss/components/navbar.scss";
-import "../scss/root.scss";
-
 import PrimaryButton from "./buttons/PrimaryButton";
 import Burger from "./burger";
 import { useAuth } from "../context/AuthContext";