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/GeneratorShop.tsx

@@ -12,7 +12,7 @@ export function GeneratorShop() {
 
   return (
     <div className="gp">
-      <div style={{ display: "flex", justifyContent: "space-between", alignItems: "center" }}>
+      <div className="flex justify-between items-center">
         <span className="gp-title" title="Achète des générateurs pour produire des têtards automatiquement">Générateurs</span>
         <span className="gp-value gp-accent-green" title="Production totale par seconde">{formatNumber(productionPerSecond)}/s</span>
       </div>
@@ -26,8 +26,8 @@ export function GeneratorShop() {
             key={gen.id}
             className={`gp-row ${canAfford ? "gp-row--active" : "gp-row--locked"}`}
           >
-            <div style={{ display: "flex", flexDirection: "column", minWidth: 0 }}>
-              <div style={{ display: "flex", alignItems: "center", gap: "0.3rem" }}>
+            <div className="flex flex-col min-w-0">
+              <div className="flex items-center gap-1">
                 <span className="gp-value">{gen.name}</span>
                 {gen.owned > 0 && (
                   <span className="gp-label gp-accent-green">x{gen.owned}</span>