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/chat_message.ts

@@ -1,33 +0,0 @@
-/**
- * 聊天历史中的单条消息 DTO
- */
-import {
-  ChatMessageSchema,
-  type ChatMessageInput,
-  type ChatMessageData,
-} from "@/data/schemas/chat/chat_message";
-
-export class ChatMessage {
-  declare readonly role: string;
-  declare readonly content: string;
-  declare readonly id: string;
-  declare readonly createdAt: string;
-
-  private constructor(input: ChatMessageInput) {
-    const data = ChatMessageSchema.parse(input);
-    Object.assign(this, data);
-    Object.freeze(this);
-  }
-
-  static from(input: ChatMessageInput): ChatMessage {
-    return new ChatMessage(input);
-  }
-
-  static fromJson(json: unknown): ChatMessage {
-    return ChatMessage.from(json as ChatMessageInput);
-  }
-
-  toJson(): ChatMessageData {
-    return ChatMessageSchema.parse(this);
-  }
-}