refactor(data): split models into separate dto and schema modules

Reorganize the data layer by separating DTOs (Data Transfer Objects) from
Zod validation schemas into distinct directory structures. Previously,
all model types were imported from a single `@/data/models` barrel
file, mixing request/response classes with validation schemas.

Changes:
- Update API files (auth, chat, metrics, user) to import DTOs from
  specific paths under `@/data/dto/*` instead of the models barrel
- Move user schema to `@/data/schemas/user/user` for type-only imports
- Create dedicated Zod schema files under `@/data/schemas/*` for
  auth and chat domain models (e.g., AppleLoginRequestSchema,
  SendMessageRequestSchema, SyncMessageSchema, SttDataSchema)
- Improve discoverability and tree-shaking by co-locating schemas
  with their respective domain areas

This is a non-functional refactor that lays the groundwork for
clearer separation between transport-layer types and runtime
validation.

src/data/schemas/auth/login_request.ts

@@ -0,0 +1,16 @@
+/**
+ * 登录请求
+ * 原始 Dart: LoginRequest (lib/data/models/auth/auth_request.dart)
+ */
+import { z } from "zod";
+
+export const LoginRequestSchema = z.object({
+  email: z.string().default(""),
+  username: z.string().default(""),
+  password: z.string(),
+  platform: z.string(),
+  guestId: z.string().default(""),
+});
+
+export type LoginRequestInput = z.input<typeof LoginRequestSchema>;
+export type LoginRequestData = z.output<typeof LoginRequestSchema>;