feat(styles): migrate design system tokens to CSS variables

Replace minimal background/foreground vars with a comprehensive design
system covering colors (background, text, brand, profile, gifts,
activities, nav), spacing, and border-radius tokens. Mirrors the
original Dart design constants under src/design/*.ts and supports
dark mode.

src/app/layout.tsx

@@ -13,8 +13,8 @@ const geistMono = Geist_Mono({
 });
 
 export const metadata: Metadata = {
-  title: "Create Next App",
-  description: "Generated by create next app",
+  title: "cozsweet",
+  description: "cozsweet frontend (Next.js)",
 };
 
 export default function RootLayout({