fix(extension): show startup failure guidance

[fuyua9]

Summary

• show an inline side panel diagnostic when task startup fails before the agent can render activity
• include the recovery steps from the maintainer comment in [Bug] Extension becomes unresponsive (service worker inactive) #452 for stalled MV3 service-worker updates
• clear the diagnostic when the user retries or navigates to settings/history

Tests

• npm run typecheck
• npm run lint
• npm run build

Refs #452

[gaomeng1900]

This should have been fixed by #596