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/AchievementCardGenerator.swift

@@ -38,6 +38,8 @@ struct AchievementSpotlightContent: ShareableContent {
 struct AchievementCollectionContent: ShareableContent {
     let achievements: [AchievementProgress]
     let year: Int
+    var sports: Set<Sport> = []  // Sports for background icons
+    var filterSport: Sport? = nil  // The sport filter applied (for header title)
 
     var cardType: ShareCardType { .achievementCollection }
 
@@ -46,6 +48,8 @@ struct AchievementCollectionContent: ShareableContent {
         let cardView = AchievementCollectionView(
             achievements: achievements,
             year: year,
+            sports: sports,
+            filterSport: filterSport,
             theme: theme
         )
 
@@ -120,9 +124,16 @@ private struct AchievementSpotlightView: View {
     let achievement: AchievementProgress
     let theme: ShareTheme
 
+    private var sports: Set<Sport> {
+        if let sport = achievement.definition.sport {
+            return [sport]
+        }
+        return []
+    }
+
     var body: some View {
         ZStack {
-            ShareCardBackground(theme: theme)
+            ShareCardBackground(theme: theme, sports: sports)
 
             VStack(spacing: 50) {
                 Spacer()
@@ -176,6 +187,8 @@ private struct AchievementSpotlightView: View {
 private struct AchievementCollectionView: View {
     let achievements: [AchievementProgress]
     let year: Int
+    let sports: Set<Sport>
+    let filterSport: Sport?
     let theme: ShareTheme
 
     private let columns = [
@@ -184,13 +197,20 @@ private struct AchievementCollectionView: View {
         GridItem(.flexible(), spacing: 30)
     ]
 
+    private var headerTitle: String {
+        if let sport = filterSport {
+            return "My \(String(year)) \(sport.rawValue) Achievements"
+        }
+        return "My \(String(year)) Achievements"
+    }
+
     var body: some View {
         ZStack {
-            ShareCardBackground(theme: theme)
+            ShareCardBackground(theme: theme, sports: sports)
 
             VStack(spacing: 40) {
                 // Header
-                Text("My \(String(year)) Achievements")
+                Text(headerTitle)
                     .font(.system(size: 48, weight: .bold, design: .rounded))
                     .foregroundStyle(theme.textColor)
 
@@ -241,9 +261,16 @@ private struct AchievementMilestoneView: View {
 
     private let goldColor = Color(hex: "FFD700")
 
+    private var sports: Set<Sport> {
+        if let sport = achievement.definition.sport {
+            return [sport]
+        }
+        return []
+    }
+
     var body: some View {
         ZStack {
-            ShareCardBackground(theme: theme)
+            ShareCardBackground(theme: theme, sports: sports)
 
             // Confetti burst pattern
             ConfettiBurst()
@@ -304,9 +331,16 @@ private struct AchievementContextView: View {
     let mapSnapshot: UIImage?
     let theme: ShareTheme
 
+    private var sports: Set<Sport> {
+        if let sport = achievement.definition.sport {
+            return [sport]
+        }
+        return []
+    }
+
     var body: some View {
         ZStack {
-            ShareCardBackground(theme: theme)
+            ShareCardBackground(theme: theme, sports: sports)
 
             VStack(spacing: 40) {
                 // Header with badge and name