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/ProgressCardGenerator.swift

@@ -53,7 +53,7 @@ private struct ProgressCardView: View {
 
     var body: some View {
         ZStack {
-            ShareCardBackground(theme: theme)
+            ShareCardBackground(theme: theme, sports: [progress.sport])
 
             VStack(spacing: 40) {
                 ShareCardHeader(