feat: redesign all share cards, remove unused achievement types, fix sport selector

Redesign trip, progress, and achievement share cards with premium
sports-media aesthetic. Remove unused milestone/context achievement card
types (only used in debug exporter). Fix gold text unreadable in light
mode. Fix sport selector to only show stroke on selected sport.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SportsTime/Export/Sharing/ShareableContent.swift

@@ -21,8 +21,6 @@ enum ShareCardType: String, CaseIterable {
     case tripSummary
     case achievementSpotlight
     case achievementCollection
-    case achievementMilestone
-    case achievementContext
     case stadiumProgress
 }
 
@@ -124,6 +122,72 @@ struct ShareTheme: Identifiable, Hashable {
     static func theme(byId id: String) -> ShareTheme {
         all.first { $0.id == id } ?? .dark
     }
+
+    // MARK: - Derived Theme Properties
+
+    /// Glass panel fill — textColor at low opacity
+    var surfaceColor: Color {
+        textColor.opacity(0.08)
+    }
+
+    /// Panel border — textColor at medium-low opacity
+    var borderColor: Color {
+        textColor.opacity(0.15)
+    }
+
+    /// Glow effect color — accentColor at medium opacity
+    var glowColor: Color {
+        accentColor.opacity(0.4)
+    }
+
+    /// Highlight gradient for accent elements
+    var highlightGradient: [Color] {
+        [accentColor, accentColor.opacity(0.6)]
+    }
+
+    /// Mid-tone color derived from gradient endpoints for richer backgrounds
+    var midGradientColor: Color {
+        gradientColors.count >= 2
+            ? gradientColors[0].blendedWith(gradientColors[1], fraction: 0.5)
+            : gradientColors.first ?? .black
+    }
+}
+
+// MARK: - Color Blending Helper
+
+extension Color {
+    /// Simple blend between two colors at a given fraction (0 = self, 1 = other)
+    func blendedWith(_ other: Color, fraction: Double) -> Color {
+        let f = max(0, min(1, fraction))
+        let c1 = UIColor(self).rgbaComponents
+        let c2 = UIColor(other).rgbaComponents
+        return Color(
+            red: c1.r + (c2.r - c1.r) * f,
+            green: c1.g + (c2.g - c1.g) * f,
+            blue: c1.b + (c2.b - c1.b) * f,
+            opacity: c1.a + (c2.a - c1.a) * f
+        )
+    }
+}
+
+private extension UIColor {
+    var rgbaComponents: (r: CGFloat, g: CGFloat, b: CGFloat, a: CGFloat) {
+        var r: CGFloat = 0
+        var g: CGFloat = 0
+        var b: CGFloat = 0
+        var a: CGFloat = 0
+
+        if getRed(&r, green: &g, blue: &b, alpha: &a) {
+            return (r, g, b, a)
+        }
+
+        var white: CGFloat = 0
+        if getWhite(&white, alpha: &a) {
+            return (white, white, white, a)
+        }
+
+        return (0, 0, 0, 1)
+    }
 }
 
 // MARK: - Share Errors