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/Core/Services/AchievementEngine.swift

@@ -33,11 +33,16 @@ final class AchievementEngine {
 
     // MARK: - Initialization
 
-    init(modelContext: ModelContext, dataProvider: AppDataProvider = AppDataProvider.shared) {
+    init(modelContext: ModelContext, dataProvider: AppDataProvider) {
         self.modelContext = modelContext
         self.dataProvider = dataProvider
     }
 
+    /// Convenience initializer using the shared data provider
+    convenience init(modelContext: ModelContext) {
+        self.init(modelContext: modelContext, dataProvider: AppDataProvider.shared)
+    }
+
     // MARK: - Public API
 
     /// Full recalculation (call after visit deleted or on app update)