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/chat/image_upload_response.ts

@@ -1,19 +0,0 @@
-/**
- * 图片上传响应
- * 原始 Dart: ImageUploadResponse (lib/data/models/chat/image_upload_response.dart)
- */
-import { z } from "zod";
-
-export const ImageUploadResponseSchema = z.object({
-  success: z.boolean().default(true),
-  imageId: z.string(),
-  thumbUrl: z.string(),
-  mediumUrl: z.string(),
-  originalUrl: z.string(),
-  width: z.number(),
-  height: z.number(),
-  bytes: z.number(),
-});
-
-export type ImageUploadResponseInput = z.input<typeof ImageUploadResponseSchema>;
-export type ImageUploadResponseData = z.output<typeof ImageUploadResponseSchema>;