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/services/api/chat_api.ts

@@ -0,0 +1,87 @@
+/**
+ * Chat API
+ *
+ * 聊天相关接口
+ * 原始 Dart: lib/data/services/api/chat_api_client.dart
+ */
+import { httpClient } from "./http_client";
+import { ApiPath } from "./api_path";
+import { ApiEnvelope, unwrap } from "./response_helper";
+import { ChatHistoryResponse } from "@/data/dto/chat/chat_history_response";
+import { ChatSendResponse } from "@/data/dto/chat/chat_send_response";
+import { ChatSyncData } from "@/data/dto/chat/chat_sync_data";
+import { ChatSyncRequest } from "@/data/dto/chat/chat_sync_request";
+import { ImageUploadResponse } from "@/data/dto/chat/image_upload_response";
+import { SendMessageRequest } from "@/data/dto/chat/send_message_request";
+import { SttData } from "@/data/dto/chat/stt_data";
+import { SyncMessage } from "@/data/dto/chat/sync_message";
+
+export class ChatApi {
+  /**
+   * 发送消息
+   */
+  async sendMessage(body: SendMessageRequest): Promise<ChatSendResponse> {
+    const env = await httpClient<ApiEnvelope<unknown>>(ApiPath.chatSend, {
+      method: "POST",
+      body: body.toJson(),
+    });
+    return ChatSendResponse.fromJson(unwrap(env) as Record<string, unknown>);
+  }
+
+  /**
+   * 获取聊天历史
+   */
+  async getHistory(limit = 50, offset = 0): Promise<ChatHistoryResponse> {
+    const env = await httpClient<ApiEnvelope<unknown>>(ApiPath.chatHistory, {
+      query: { limit, offset },
+    });
+    return ChatHistoryResponse.fromJson(
+      unwrap(env) as Record<string, unknown>
+    );
+  }
+
+  /**
+   * 语音转文字（multipart 上传）
+   */
+  async speechToText(audio: Blob | File): Promise<SttData> {
+    const formData = new FormData();
+    formData.append("audio", audio);
+    const env = await httpClient<ApiEnvelope<unknown>>(ApiPath.chatStt, {
+      method: "POST",
+      body: formData,
+    });
+    return SttData.fromJson(unwrap(env) as Record<string, unknown>);
+  }
+
+  /**
+   * 同步游客消息
+   */
+  async syncMessages(messages: SyncMessage[]): Promise<ChatSyncData> {
+    const body = ChatSyncRequest.from({ messages });
+    const env = await httpClient<ApiEnvelope<unknown>>(ApiPath.chatSync, {
+      method: "POST",
+      body: body.toJson(),
+    });
+    return ChatSyncData.fromJson(unwrap(env) as Record<string, unknown>);
+  }
+
+  /**
+   * 上传图片
+   */
+  async uploadImage(image: Blob | File): Promise<ImageUploadResponse> {
+    const formData = new FormData();
+    formData.append("image", image);
+    const env = await httpClient<ApiEnvelope<unknown>>(
+      ApiPath.chatUploadImage,
+      { method: "POST", body: formData }
+    );
+    return ImageUploadResponse.fromJson(
+      unwrap(env) as Record<string, unknown>
+    );
+  }
+}
+
+/**
+ * 全局单例
+ */
+export const chatApi = new ChatApi();