fix: resolve compiler warnings across codebase

- PaywallView: remove unnecessary nil coalescing for currencyCode
- GameDAGRouter: change var to let for immutable compositeKeys
- GamesHistoryRow/View: add missing wnba and nwsl switch cases
- VisitDetailView: fix unused variable in preview
- AchievementEngine: use convenience init to avoid default parameter warning
- ProgressCardGenerator: use method overload instead of default parameter
- StadiumProximityMatcher: extract constants to ProximityConstants enum

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

SportsTime/Features/Progress/Views/Components/GamesHistoryRow.swift

@@ -51,7 +51,8 @@ struct GamesHistoryRow: View {
         case .nhl: return "hockey.puck"
         case .nfl: return "football"
         case .mls: return "soccerball"
-        @unknown default: return "sportscourt"
+        case .wnba: return "basketball"
+        case .nwsl: return "soccerball"
         }
     }
 }

SportsTime/Features/Progress/Views/GamesHistoryView.swift

@@ -140,7 +140,8 @@ private struct SportChip: View {
         case .nhl: return "hockey.puck"
         case .nfl: return "football"
         case .mls: return "soccerball"
-        @unknown default: return "sportscourt"
+        case .wnba: return "basketball"
+        case .nwsl: return "soccerball"
         }
     }
 }

SportsTime/Features/Progress/Views/VisitDetailView.swift

@@ -522,7 +522,7 @@ extension VisitSource {
 // MARK: - Preview
 
 #Preview {
-    let stadium = Stadium(
+    let _ = Stadium(
         id: "stadium_preview_oracle_park",
         name: "Oracle Park",
         city: "San Francisco",