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/dto/chat/sync_message.ts

@@ -0,0 +1,32 @@
+/**
+ * 待同步的单条消息 DTO
+ */
+import {
+  SyncMessageSchema,
+  type SyncMessageInput,
+  type SyncMessageData,
+} from "@/data/services/schemas/chat/sync_message";
+
+export class SyncMessage {
+  declare readonly role: string;
+  declare readonly content: string;
+  declare readonly timestamp: string;
+
+  private constructor(input: SyncMessageInput) {
+    const data = SyncMessageSchema.parse(input);
+    Object.assign(this, data);
+    Object.freeze(this);
+  }
+
+  static from(input: SyncMessageInput): SyncMessage {
+    return new SyncMessage(input);
+  }
+
+  static fromJson(json: unknown): SyncMessage {
+    return SyncMessage.from(json as SyncMessageInput);
+  }
+
+  toJson(): SyncMessageData {
+    return SyncMessageSchema.parse(this);
+  }
+}