Fix Avalonia startup display sizing and system switch state

[highbyte]

Summary

• Prevent Avalonia startup display from briefly sizing to the 320x200 fallback before the default system is selected.
• Add config-derived screen info for C64, VIC-20, and Generic systems before an emulator instance exists.
• Fix system switch notifications so invalid-system validation state/menus do not leak into the next selected system.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud