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/Planning/Engine/GameDAGRouter.swift

@@ -324,7 +324,7 @@ enum GameDAGRouter {
         }
 
         // Round-robin from composite buckets
-        var compositeKeys = Array(byComposite.keys).sorted()
+        let compositeKeys = Array(byComposite.keys).sorted()
         var indices: [String: Int] = [:]
         while selected.count < maxCount && !compositeKeys.isEmpty {
             var addedAny = false