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- Delete PersistenceActor.swift placeholder (superseded by PersistenceController.swift)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/Services/CoreML/ModelManager.swift

@@ -1,5 +1,6 @@
 import CoreML
 import Vision
+import FirebaseCore
 import FirebaseStorage
 
 struct SharedEngineResources {
@@ -7,6 +8,10 @@ struct SharedEngineResources {
 }
 
 // MARK: - MODEL MANAGER (OTA Updates)
+// Downloads updated .mlmodel files from Firebase Storage and compiles them to Documents/models/.
+// IMPORTANT: Each engine (FoilEngine, ConditionEngine, etc.) loads its model via a `static var`
+// that is evaluated once at class-load time. A downloaded update will NOT take effect until the
+// app is restarted. Inform users via release notes when a model update has been pushed OTA.
 class ModelManager {
     static let shared = ModelManager()
     private let defaults = UserDefaults.standard