Use dark mode for built-in diff viewer

[stuartd]

The built-in WPF viewer was still using Windows light system brushes while the tray menu, About window, and shortcut window were already dark.

This change:

• applies ClipDiff's existing dark palette to the viewer, headers, controls, lists, and selectable diff text;
• preserves the red/green diff highlighting against the dark background;
• enables the native immersive-dark title bar, including the pre-20H1 fallback already used elsewhere in ClipDiff.

Validation:

• git diff --check passes.
• The workspace does not provide the .NET SDK, so compilation and tests are left to the repository's Windows CI.