feat(ui): add sport backgrounds to share cards, achievement filtering, and wizard validation

- Add ShareCardSportBackground with floating sport icons for share cards
- Share cards now show sport-specific backgrounds (single or multiple sports)
- Achievement collection share respects sport filter selection
- Add ability to share individual achievements from detail sheet
- Trip wizard ReviewStep highlights missing required fields in red
- Add FieldValidation model to TripWizardViewModel

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

SportsTime/Export/Sharing/ShareCardComponents.swift

@@ -13,13 +13,18 @@ import UIKit
 
 struct ShareCardBackground: View {
     let theme: ShareTheme
+    var sports: Set<Sport>? = nil
 
     var body: some View {
-        LinearGradient(
-            colors: theme.gradientColors,
-            startPoint: .top,
-            endPoint: .bottom
-        )
+        if let sports = sports, !sports.isEmpty {
+            ShareCardSportBackground(sports: sports, theme: theme)
+        } else {
+            LinearGradient(
+                colors: theme.gradientColors,
+                startPoint: .top,
+                endPoint: .bottom
+            )
+        }
     }
 }