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/utils/device_identifier.ts

@@ -6,7 +6,7 @@
  *
  * 注：仅在浏览器环境可用；SSR/Server Components 中调用会抛错。
  */
-import { authStorage } from "../../storage/auth_storage";
+import { authStorage } from "../data/storage/auth_storage";
 
 export class DeviceIdentifier {
   private cachedId: string | null = null;