Make the brand purple the accent color and exercise Done check

Populate the previously-empty AccentColor asset (iOS + watch) with the
logo purple — a deep shade in light mode, brightened for dark mode and
the watch's black background. The exercise Done check now uses that
accent color and the in-progress indicator reads as a neutral gray, on
both iPhone and Apple Watch.

Workouts/Views/Common/CheckboxStatus.swift

@@ -17,9 +17,9 @@ enum CheckboxStatus {
 
     var color: Color {
         switch self {
-        case .checked: .green
+        case .checked: .accentColor
         case .unchecked: .gray
-        case .intermediate: .yellow
+        case .intermediate: .gray
         case .cancelled: .red
         }
     }