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/schemas/auth/send_code_request.ts

@@ -1,12 +0,0 @@
-/**
- * 发送验证码请求
- * 原始 Dart: SendCodeRequest (lib/data/models/auth/auth_request.dart)
- */
-import { z } from "zod";
-
-export const SendCodeRequestSchema = z.object({
-  email: z.string(),
-});
-
-export type SendCodeRequestInput = z.input<typeof SendCodeRequestSchema>;
-export type SendCodeRequestData = z.output<typeof SendCodeRequestSchema>;