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/Color+Codable.swift

@@ -99,6 +99,40 @@ extension String {
     }
 }
 
+// MARK: - Luminance & Contrast
+
+extension Color {
+    /// Calculate relative luminance using WCAG formula
+    /// Returns value between 0 (darkest) and 1 (lightest)
+    var luminance: Double {
+        guard let components = cgColor?.components, components.count >= 3 else {
+            return 0.5 // Default to mid-gray if can't determine
+        }
+
+        let r = components[0]
+        let g = components[1]
+        let b = components[2]
+
+        // Apply gamma correction per WCAG 2.0
+        func adjust(_ component: CGFloat) -> Double {
+            let c = Double(component)
+            return c <= 0.03928 ? c / 12.92 : pow((c + 0.055) / 1.055, 2.4)
+        }
+
+        return 0.2126 * adjust(r) + 0.7152 * adjust(g) + 0.0722 * adjust(b)
+    }
+
+    /// Returns true if the color is considered "light" (needs dark text for contrast)
+    var isLight: Bool {
+        luminance > 0.5
+    }
+
+    /// Returns black or white depending on which provides better contrast
+    var contrastingTextColor: Color {
+        isLight ? .black : .white
+    }
+}
+
 extension Color: @retroactive RawRepresentable {
     // TODO: Sort out alpha
     public init?(rawValue: Int) {