fix: apply high contrast colors when system theme is selected

[afonsojramos]

Summary

Fixes #2489

When "System" theme is selected with "Increase contrast" enabled, the high contrast color schemes were not being applied. The app would fall back to default light/dark schemes instead of light_high_contrast/dark_high_contrast.

This adds high contrast fallback constants and uses them when the system theme returns null for the color scheme.

Test plan

• Select "System" theme in Settings → Appearance
• Enable "Increase contrast"
• Verify high contrast colors are applied in both light and dark system modes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
85.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[afonsojramos]

@setchy found this one out testing the tauri branch, but found out that it actually already existed 🙈