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/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",