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/app/sidebar/components/sidebar-screen.tsx

@@ -44,7 +44,6 @@ export function SidebarScreen() {
 
   // 等价 Dart: BlocConsumer<UserBloc> 的 listener（profile_view.dart:24-34）
   // 仅在 currentUser 由非 null 跳到 null 时触发清理
-  // 注：chat 机器不再需要 `ChatAuthStatusChanged` 通知（已删）—— chat-screen
   //     通过 `useAuthState()` 主动订阅 loginStatus；这里只需重置 auth + 跳 /chat
   const prevUserRef = useRef<UserView | null>(user.currentUser);
   useEffect(() => {