refactor(data): consolidate data layer under services namespace

- Move src/data/{api,dto,schemas} directories to src/data/services/*
- Update all relative imports across the codebase to reference new paths
- Use CSS color tokens for splash button gradient in splash-button.module.css
- Add responsive sizes attribute to splash background image
- Add JSDoc documentation to splash-background component referencing original Dart implementation

src/data/storage/user/user_storage.ts

@@ -15,7 +15,7 @@
  *   - 单例挂在 class 静态字段，HMR 复用安全
  */
 
-import { UserSchema, type UserData } from "@/data/schemas/user/user";
+import { UserSchema, type UserData } from "@/data/services/schemas/user/user";
 import { LocalStorage } from "../local_storage";
 import { type Result as ResultT } from "@/utils/result";
 import { StorageKeys } from "../storage_keys";