fix(auth): stop parsing logout response body (avoids ZodError on null data)

Root cause: backend `/auth/logout` returns `{ success: true, data: null }`
(no business payload for a fire-and-forget endpoint). The previous
`AuthApi.logout` ran `unwrap(env)` (which passes null through, since
null is defined) and then `LogoutResponse.fromJson(null)`, which
Zod-parses as `z.object(...)` and throws "Invalid input: expected
object, received null".

The error was caught in `AuthRepository.logout` with
`console.warn(... clearing local anyway)` — logout actually still
worked (local clear ran), but a noisy red ZodError hit the console on
every logout click.

Fix: align `AuthApi.logout` with the existing fire-and-forget pattern
used by `register` and `sendCode` — call `httpClient` and ignore the
response body. Drop the now-unused `LogoutResponse` import.

`AuthRepository.logout` already discards the return value, so the
`Promise<LogoutResponse> → Promise<void>` signature change is safe.

Bundled in this commit (unrelated):
- chat-machine.actors.ts / user-machine.ts: removed stale design-doc
  comment blocks (IDE/linter cleanup)
- user-machine.actors.ts: removed redundant `userStorage.clearUserData()`
  from `userLogoutActor` — `authRepo.logout` already clears local
  user data, so this was a no-op duplicate.
- sidebar-screen.tsx: removed one stale comment line.

src/stores/user/user-machine.actors.ts

@@ -70,5 +70,4 @@ export const userFetchActor = fromPromise<UserView | null>(async () => {
  */
 export const userLogoutActor = fromPromise<void>(async () => {
   await authRepo.logout();
-  await userStorage.clearUserData();
 });

src/stores/user/user-machine.ts

@@ -1,19 +1,5 @@
 /**
  * User 状态机（XState v5）
- *
- * 原始 Dart: lib/ui/user/bloc/user bloc + user state + user event
- *
- * 设计要点：
- *   - XState v5 `setup({...}).createMachine({...})` 声明式 API
- *   - HTTP 操作用 `fromPromise` actor（一次性 Promise）→ 见 `user-machine.actors.ts`
- *   - 纯函数 / 数据加载 → 见 `user-machine.helpers.ts`
- *   - 同步状态更新用 `assign` actions（inline 在 setup() 中）
- *   - 保持事件类型名与原 Dart 一致
- *
- * 分层：
- *   - helpers: 无 XState 依赖（DTO 映射 + 数据加载）
- *   - actors:  依赖 helpers（异步 actor 实现）
- *   - machine: 依赖 actors（XState setup + 状态图）
  */
 import { setup, assign } from "xstate";