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

[hotic]

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

TUI already has confirmation for delete:

Request: Add similar two-step confirmation to WebUI.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• bug(webui): archived session "flashes back" due to SSE race condition #7882: bug(webui): archived session "flashes back" due to SSE race condition - Also addresses issues with archiving sessions in the WebUI sidebar and proposes using the same delete logic as TUI

Feel free to ignore if none of these address your specific case.