fix: avoid background HID polling resetting system idle timers

[ArtisticZhao]

Summary

This changes Mouser's background HID polling so it no longer keeps resetting the OS idle timer while the app is hidden or the user is inactive.

The issue appears to come from periodic HID++ reads, especially SmartShift polling, being interpreted by the OS as user activity. That prevents display timeout and system sleep even when the mouse and keyboard are untouched.

Changes

• Gate background battery / SmartShift HID reads behind frontend visibility.
• Skip background HID polling once the system has been idle for a grace period.
• Avoid repeated background polls unless there has been real user activity since the last poll.
• Increase background polling intervals to reduce unnecessary HID traffic.
• Track QML window visibility, including minimized state, and pass it to the engine.
• Add tests covering hidden frontend, idle system state, and repeated polling without user activity.

Related

Related to #178.

The discussion there identified periodic HID++ reads, especially SmartShift polling, as a likely cause of the OS idle timer being reset while Mouser runs in the background. This PR makes those background reads visibility- and idle-aware, and I manually verified the macOS sleep behavior after building the updated app locally.