test(main): cover updater fallback, ipc sender guard, log sanitizer, sftp timeouts

- updater-fallback.mjs (82 assertions): GitHub probe fallback to Gitea,
  timeout budgets, in-flight check never stuck in 'checking'; updater.ts
  gains a setUpdateTimeouts test seam, electron-updater aliased to a stub
- ipc-guard.mjs (43): trusted-frame guard exercised through real
  registerIpc handlers with forged senderFrames; electron-stub now records
  registrations via globalThis so bundle and test share one instance
- log-sanitizer.mjs (71): CSI/OSC/charset state machine, alt-screen fold,
  byte-split fuzz equal to whole-chunk output; fixes a wrong comment
- sftp-timeout.mjs (39): per-op timeouts (metadata 30s, transfer chunk 60s,
  open 10s) evict half-dead channels with one retry, slow-but-progressing
  transfers untouched, late rejections never unhandled
- reservedAccelerators.ts: single pure isReservedAccelerator shared by the
  settings recorder and applyGlobalShortcut (the two tables had drifted —
  main now also refuses Ctrl+=/-/0/PgUp/PgDn legacy values); 56 assertions
- ssh-loopback.mjs loads the real ssh.ts via a bundle (50 assertions):
  TOFU pinning, fail-closed stores, auth gate, connect budget

Offline suite grows 13 -> 17. Renderer test framework evaluated: not
introducing vitest/jsdom; pure logic keeps being extracted and tested
through the existing bundle harness.

src/shared/i18n/dicts/zh-CN/main.ts

@@ -35,6 +35,7 @@ const main: Record<string, string> = {
   'main.sftp.invalidUidGid': '非法 uid/gid',
   'main.sftp.commandTimeout': '命令执行超时',
   'main.sftp.openTimeout': 'SFTP 通道打开超时',
+  'main.sftp.opTimeout': 'SFTP 操作无响应（{seconds} 秒），连接可能已中断，请重新连接会话后重试',
   'main.sftp.commandExitCode': '命令退出码 {code}',
   'main.sftp.cancelled': '已取消',
   'main.sftp.localNotAllowed': '本地路径未被授权: {path}。请通过「选择文件 / 选择目录」对话框重新选择。',