fix: Resolve all audit issues from project readiness review

Blockers:
- IYmtgTests: replace ScannerViewModel() (no-arg init removed) with CollectionViewModel() in testViewModelFiltering and testPortfolioCalculation
- IYmtgTests: fix property access — scannedList, librarySearchText, filteredList, portfolioValue all live on CollectionViewModel, not ScannerViewModel; inject test data after async init settles

Major:
- ContentView: update .onChange(of:) to two-parameter closure syntax (iOS 17 deprecation)
- ModelManager: add missing import FirebaseCore so FirebaseApp.app() resolves explicitly
- CollectionViewModel.deleteCard: call CloudEngine.delete(card:) to remove Firebase entry when a card is deleted (prevents data accumulation)
- CloudEngine: remove never-called batchUpdatePrices() — full backup already handled by backupAllToFirebase()

Minor:
- CardDetailView: move to Features/CardDetail/CardDetailView.swift; remove from ContentView.swift
- Delete PersistenceActor.swift placeholder (superseded by PersistenceController.swift)
- AppConfig.validate(): broaden placeholder-email guard to catch empty strings and common fake domains
- ModelManager: document OTA restart requirement in code comment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

IYmtg_App_iOS/Features/Collection/CollectionViewModel.swift

@@ -272,6 +272,7 @@ class CollectionViewModel: ObservableObject {
         if let idx = scannedList.firstIndex(where: { $0.id == card.id }) {
             let fileName = scannedList[idx].imageFileName
             Task { await ImageManager.delete(name: fileName) }
+            Task { await CloudEngine.delete(card: card) }
             scannedList.remove(at: idx)
             self.saveCollectionAsync()
         }