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/chat_sync_data.ts

@@ -1,13 +0,0 @@
-/**
- * 消息同步响应
- * 原始 Dart: ChatSyncData (lib/data/models/chat/chat_sync.dart)
- */
-import { z } from "zod";
-
-export const ChatSyncDataSchema = z.object({
-  syncedCount: z.number(),
-  totalHistory: z.number(),
-});
-
-export type ChatSyncDataInput = z.input<typeof ChatSyncDataSchema>;
-export type ChatSyncDataData = z.output<typeof ChatSyncDataSchema>;