feat(app): add two-step confirmation for archive session

Single-click archive in sidebar can cause accidental data loss.

TUI already has confirmation for delete:

<img width="687" height="162" alt="Image" src="https://github.com/user-attachments/assets/2ee4ded4-6911-41e5-b987-be112fe2cd7d" />



Request: Add similar two-step confirmation to WebUI.