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/dto/chat/image_upload_response.ts

@@ -1,37 +0,0 @@
-/**
- * 图片上传响应 DTO
- */
-import {
-  ImageUploadResponseSchema,
-  type ImageUploadResponseInput,
-  type ImageUploadResponseData,
-} from "@/data/schemas/chat/image_upload_response";
-
-export class ImageUploadResponse {
-  declare readonly success: boolean;
-  declare readonly imageId: string;
-  declare readonly thumbUrl: string;
-  declare readonly mediumUrl: string;
-  declare readonly originalUrl: string;
-  declare readonly width: number;
-  declare readonly height: number;
-  declare readonly bytes: number;
-
-  private constructor(input: ImageUploadResponseInput) {
-    const data = ImageUploadResponseSchema.parse(input);
-    Object.assign(this, data);
-    Object.freeze(this);
-  }
-
-  static from(input: ImageUploadResponseInput): ImageUploadResponse {
-    return new ImageUploadResponse(input);
-  }
-
-  static fromJson(json: unknown): ImageUploadResponse {
-    return ImageUploadResponse.from(json as ImageUploadResponseInput);
-  }
-
-  toJson(): ImageUploadResponseData {
-    return ImageUploadResponseSchema.parse(this);
-  }
-}