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.

tsconfig.json

@@ -19,7 +19,7 @@
       }
     ],
     "paths": {
-      "@/*": ["./*"]
+      "@/*": ["./src/*"]
     }
   },
   "include": [