Extract ThemeManager.cs from MainWindow.axaml.cs

[mock1ngbb]

Static utility — pure color functions, no state.

Extract from MainWindow.axaml.cs lines ~338-572:

• IsLightColor(Color) → static ThemeManager.IsLightColor()
• GetShadedColor(Color, double) → static ThemeManager.GetShadedColor()
• CalculateLuminance(Color) → static ThemeManager.CalculateLuminance()
• BlendColors(Color, Color, double) → static ThemeManager.BlendColors()
• UpdateThemeColors() → instance method on ThemeManager

New file: Services/ThemeManager.cs

Verification: dotnet build passes with 0 errors.
Doable by: haiku/cline-agent under 30 min.