feat(lock): main-window lock screen; harden env gating, sandbox and ssh teardown

Lock screen (main-window overlay, no second window):
- scrypt password verifier in <userData>/lock.json (per-write salt,
  timingSafeEqual); salt/hash/password never leave the main process
- lock now / idle auto-lock / lock at startup, growing failure cooldown,
  lock flags persisted so a quit-and-relaunch cannot bypass the lock
- locked shell and body portals go inert while sessions keep running;
  menu accelerators (reload, DevTools, zoom) are swallowed while locked
- settings gains a Lock tab; all copy in zh-CN/zh-TW/en/ja

Security and stability:
- packaged builds ignore ELECTRON_RENDERER_URL / OT_UPDATE_URL (devEnv)
- renderer preload runs with sandbox: true
- unreadable known_hosts store fails closed instead of being overwritten
- connect-time secrets gated by the bookmark's auth method (connectPromptFor)
- ssh stream teardown is idempotent: PTY_EXIT broadcasts exactly once
- sysinfo polling is refcounted for split panes (forceStopPolling on close)
- session-log index entries are path-contained; settings store writes
  atomically with EPERM/EBUSY retry
- sync-changelog tolerates CRLF checkouts (was a silent no-op)
- retry ssh2 host-key generation (flaky malformed key, ~1/500)

Tests: lock-store + lock-controller suites; transport-death PTY_EXIT e2e;
GitHub Actions CI (typecheck + 10 offline tests + build)

src/shared/i18n/dicts/en/main.ts

@@ -17,6 +17,7 @@ const main: Record<string, string> = {
 
   'main.ssh.connectTimeout': 'Connection timed out ({host}:{port})',
   'main.ssh.saveFingerprintFailed': 'Failed to save the host fingerprint: {detail}',
+  'main.ssh.knownHostsUnreadable': 'The known-hosts file (ssh_known_hosts.json) exists but could not be read. The connection was refused to protect the pinned fingerprints. Repair or delete the file and try again.',
   'main.ssh.hostKeyRejected': 'The user rejected the host key',
   'main.ssh.connectFailed': 'Connection failed {host}:{port}: {detail}',
   'main.ssh.shellOpenFailed': 'Could not open the SSH shell ({host}:{port}): {detail}',