feat(window): support multiple Harness session windows

[anticipate218]

Summary

• add a registry for every desktop-created Harness window so privileged IPC remains main-frame scoped without being tied to one singleton window
• add Window → New Window / CmdOrCtrl+Shift+N, a matching Windows titlebar menu entry, and user-initiated second-launch behavior for opening another session window
• route zoom, reload, theme, menu, About, and directory-picker actions to the window that issued them
• keep one persisted primary window state, cascade additional windows, and transfer state ownership if a surviving window is promoted
• close secondary windows before shared Harness runtime/Profile transitions so stale origins, auth tokens, or Normal/Safe Mode pages cannot survive a restart
• keep migration/plugin-removal boot confirmation restricted to the current primary main frame

Verification

Passed locally on Windows:

• npm run typecheck
• npm run build
• npm test -- test/windows-titlebar.test.ts test/app-window-registry.test.ts test/window-state.test.ts (13 tests)
• git diff --check

Also ran the complete npm test suite: 934 tests passed and 8 skipped; 21 tests failed for environment/resource reasons unrelated to these window changes (missing python3 command, Windows symlink privileges, local mobile-bridge resource/port failures, pnpm build-approval environment, and a PPT hook timeout).

Manual validation still recommended

• open several session windows on Windows and macOS and confirm independent session navigation
• restart Harness and switch into/out of Safe Mode with secondary windows open
• verify Windows per-window titlebar menus and close-to-tray behavior