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/app/splash/components/splash-button.module.css

@@ -34,10 +34,11 @@
   padding: 0 30px;
   border: 0;
   border-radius: 24px;
+  /* 渐变色引用 tokens/colors.css 中定义的 token */
   background: linear-gradient(
     to right,
-    #fff96ade,
-    #fff657a0
+    var(--color-button-gradient-start),
+    var(--color-button-gradient-end)
   );
   color: #ffffff;
   font-weight: 700;