fix: resetting bg color now also resets foreground/accent color

[lacymorrow]

Summary

• When a user resets the app background color (appBgColor) to its default/unset state, the foreground/accent color (appHighlightColor) was left intact — causing buttons to become invisible against the default background
• Fix ties the highlight color to the bg color: clearing bg also clears highlight in syncSettings
• Standalone highlight-color (without a custom bg) is no longer a supported state since the two are visually coupled

Root cause

In syncSettings (src/main/crossover.js), appHighlightColor was applied independently of appBgColor. If bg was reset but highlight was still set to e.g. white, the buttons rendered as white-on-white against the system default background.

Test plan

• Set a custom App Background Color and App Icon Color in Settings → System Settings
• Click the color reset/clear button on App Background Color
• Confirm: App Icon Color is also cleared and buttons are visible
• Confirm: Setting both colors together still works correctly
• Confirm: Theme changes (light/dark/system) still reset both colors as before

Closes #3

[gemini-code-assist]

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[lacymorrow]

Closing: the bg/highlight color fix was already incorporated into main via #503 (docs: use real app screenshots) which squashed in this fix. No action needed.