Fix portable updater relaunch for v0.2.3

[oshtz]

What changed

• schedule the Windows portable replacement helper through Electron's post-exit relaunch lifecycle
• add a real Windows Electron regression fixture for replacement, relaunch, and cleanup
• bump Motif to 0.2.3

Root cause

The previous updater spawned PowerShell as a detached child and immediately quit. On Windows the detached helper exited before running, so Motif closed after download without replacing or reopening.

Validation

• npm run lint
• npm run check:desktop-security
• npm run check:release
• npm test (35 tests)
• npm run smoke:e2e
• npm run desktop:pack
• npm run desktop:smoke