feat: system Stats + CI fix + v0.4.2

[imnakul]

Summary

• add a new Stats settings page using the existing Venu visual system
• add a new Stats notch slide with compact collapsed metrics and a polished expanded view
• show CPU utilization, physical RAM usage, Windows GPU-engine activity, and battery / AC power state
• throttle sampling to about once per second and only drive notch refreshes while Stats is active
• fix the Windows CI autostart registry failure by creating the standard HKCU Run key when it does not yet exist
• bump Venu to v0.4.2 in Cargo.toml, Cargo.lock, and the Inno Setup fallback version

Stats implementation

• CPU: Win32 GetSystemTimes
• RAM: GlobalMemoryStatusEx
• Power: GetSystemPowerStatus
• GPU: Windows PDH GPU Engine(*)\Utilization Percentage; gracefully shows unavailable when a driver/session does not expose it
• no extra third-party dependency is added
• telemetry is local only; nothing is transmitted

CI fix

GitHub's clean Windows runner can lack HKCU\Software\Microsoft\Windows\CurrentVersion\Run. The write path now uses RegCreateKeyW, which safely opens the existing key or creates it when absent. The real registry round-trip test remains enabled.

After merge, the existing release workflow should detect the Cargo.toml version bump and publish v0.4.2 automatically.