fix(main): PTY_EXIT 补偿查询、SSH 飞行连接登记与双窗口守卫

- 新增 SESSION_STATE 查询通道（契约先行）：PTY_EXIT 只广播一次不重放，
  绑定晚于退出的窗格订阅后补查一次退出状态，快速退出的 shell 不再留下
  永远空白、无死亡遮罩的窗格（退出码登记表上限 512 条 FIFO）
- openSession 在 connectSsh 返回前登记 pendingOpens（owner + onClient
  最早引用），killPtysByOwner/killAllPtys 可中止飞行中的握手，renderer
  崩溃后不再留下无主孤儿 SSH 会话
- whenReady 的 createWindow 加已有窗口守卫，堵住与 second-instance
  showOrCreate 的双窗口竞态
- killAllPtys 补 replayBuffers.clear()，与 killSession 拆卸清单对齐
- promptUser 注释 30s 改为实际 120s；pty.ts 顺带注入 zmodem 背压钩子
  （见下一提交）

src/shared/api.ts

@@ -6,7 +6,8 @@ import type {
   PtyDataEvent,
   PtyExitEvent,
   SessionOpenResult,
-  SessionSnapshot
+  SessionSnapshot,
+  SessionStateResult
 } from './ipc'
 import type { AppSettings } from './settings'
 import type { ReleaseNote, UpdateState } from './ipc'
@@ -36,6 +37,13 @@ export interface AppApi {
   openSession(opts: SessionOpenOptions): Promise<SessionOpenResult>
   /** output a session produced before this renderer subscribed (capped ring buffer) */
   getSessionReplay(id: string): Promise<string>
+  /**
+   * Compensating query for a missed PTY_EXIT: that broadcast fires exactly once
+   * and is never replayed, so a pane subscribing after its shell died would
+   * wait forever. `exited` (with `exitCode`) is the death state PTY_EXIT would
+   * have set.
+   */
+  getSessionLiveState(id: string): Promise<SessionStateResult>
   writePty(id: string, data: string): void
   resizePty(id: string, cols: number, rows: number): void
   killPty(id: string): void