fix(web): discard unsaved profile changes before navigation

[youngbeom-shin]

Summary

• discard unsaved agent profile changes before blocked navigation proceeds
• allow the newly selected agent profile to load after the user confirms leaving
• add regression coverage for switching between agent profiles with an unsaved model change

[youngbeom-shin]

Fixes:

1. When an agent profile has unsaved changes, confirming the leave prompt now correctly switches to the selected agent. The sidebar selection and displayed profile remain synchronized.
2. Fixed the confirmation dialog appearing twice. Only one confirmation is now required to complete the navigation.
3. Added regression coverage for switching agents after making an unsaved model change.

Verification:

• Unsaved changes correctly trigger the leave confirmation.
• Clicking “Confirm” once successfully switches agents.
• The newly selected agent’s profile loads correctly.
• Clicking “Cancel” keeps the current profile and unsaved changes intact.
• Targeted tests and the complete frontend check pass.

Test recording:

2026-08-07.17.14.44.mov