Migrate idle timeout checker from Control Panel to WebUI

## Summary

Move the idle timeout checker (session timeout checker) management from the Control Panel into the Backend.AI WebUI, so that it can be configured directly from the WebUI instead of requiring the separate Control Panel.

## Background

The idle timeout checker configuration currently lives in the Control Panel. To consolidate administrative functionality in a single place and improve the operator experience, this management UI should be provided in the WebUI.

This follows the same direction as the earlier effort to migrate the policy menu from the Control Panel to the WebUI (see FR-1950).

## Scope

- Provide a WebUI screen/section to view and edit idle timeout checker settings currently managed in the Control Panel.
- Ensure parity with the existing Control Panel behavior (network traffic idle checker, utilization idle checker, grace period / idle timeout / time window, etc.).

## Notes

- Created without prior code investigation; exact UI placement and backend wiring to be detailed during planning/implementation.

JIRA Issue: FR-3231

Start by reviewing the existing idle timeout checker management in the Control Panel and the WebUI policy menu migration referenced as FR-1950. Map the current settings and behavior before deciding the WebUI placement and backend wiring; done means the WebUI provides equivalent viewing and editing for all listed idle-checker settings.

Written by the indexing model from the issue text.