Description

When the System theme is selected and Increase contrast is enabled, the high contrast color scheme is not applied. The app falls back to the default light/dark schemes instead of light_high_contrast/dark_high_contrast.

Steps to Reproduce

1. Open Settings → Appearance
2. Select System theme
3. Enable Increase contrast
4. Observe that the UI does not show high contrast colors

Expected Behavior

When System theme is selected with high contrast enabled:

• In light mode: light_high_contrast scheme should be applied
• In dark mode: dark_high_contrast scheme should be applied

Actual Behavior

The app uses default light and dark schemes regardless of the high contrast setting.

Root Cause

In src/renderer/utils/theme.ts, the mapThemeModeToColorScheme() function returns null for Theme.SYSTEM unconditionally (line 53), bypassing the high contrast logic entirely.

Proposed Fix

Add a new helper function that handles the System theme case by returning both day and night schemes with proper high contrast support.