Improve dark Compare and inactive tab visibility

[Leoncl2025]

Summary

• Increase dark-mode Compare line fills from about 15% to 35% opacity so added and deleted rows remain visible without hiding syntax text.
• Use primary text color for Compare metrics so counts remain readable in the dark toolbar.
• Generate active and inactive editor-tab title colors from current appearance tokens, while preserving custom.css as the final override.

Validation

• Full CTest suite: 4/4 passed.
• Added WCAG composite contrast checks requiring at least 1.35:1 for dark added/deleted line fills.
• Added a real ADS CDockWidgetTab / CElidingLabel regression test verifying the inactive title resolves to the dark secondary text color.
• git diff --check passed.

Test build

Q:\src\github\notepad-sharp\out\build\windows-x64-dark-visibility\NotepadSharp.exe