Add dynamic text contrast for mood colors and make theme preview full screen

- Add WCAG-compliant luminance calculation to Color extension
- Add contrastingTextColor method to MoodTints for automatic black/white text
- Update 12+ entry styles to use dynamic text colors instead of hardcoded white
- Change theme preview sheet to full screen presentation

Text now automatically switches between black and white based on background
brightness, fixing readability issues on light mood colors like yellow (Good).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Shared/Models/MoodTintable.swift

@@ -53,7 +53,12 @@ enum MoodTints: Int, CaseIterable {
             return PastelTint.secondary(forMood: mood)
         }
     }
-    
+
+    /// Returns black or white text color based on the mood's background luminance
+    func contrastingTextColor(forMood mood: Mood) -> Color {
+        color(forMood: mood).contrastingTextColor
+    }
+
     var moodTints: MoodTintable.Type {
         switch self {
         case .Default: