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/chat/chat-machine.actors.ts

@@ -1,18 +1,5 @@
 /**
  * Chat 状态机：4 个 XState actor
- *
- * 从 `chat-machine.ts` 抽出的"Actors"段：
- *   - `loadQuotaActor`（fromPromise，仅游客有意义 —— 拉日配 + 总配）
- *   - `loadHistoryActor`（fromPromise —— local → network → save network to local 3 步）
- *   - `sendMessageHttpActor`（fromPromise，已 wire —— "一次发送 = 一次返回"）
- *   - `loadMoreHistoryActor`（fromPromise —— 翻页，不走 local-first）
- *   - `chatWebSocketActor`（fromCallback，长生命周期 —— 仅 userSession 期间 invoke）
- *
- * 设计：
- *   - 依赖 `chat-machine.helpers.ts`（纯函数 + 数据加载）
- *   - 依赖 `createChatWebSocket`（`@/core/net/chat-websocket`）
- *   - 保留全链日志（用户要求"不要将其删除"）
- *   - 命名约定：`[chat-machine]` 前缀 —— 与 `chat-machine.ts` 的日志对齐
  */
 
 import { fromPromise, fromCallback } from "xstate";