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- Delete PersistenceActor.swift placeholder (superseded by PersistenceController.swift)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/Cloud/CloudEngine.swift

@@ -46,25 +46,4 @@ class CloudEngine {
         } catch { print("Cloud Delete Error: \(error)") }
     }
 
-    static func batchUpdatePrices(cards: [SavedCard]) async {
-        guard let db = db, let uid = Auth.auth().currentUser?.uid else { return }
-        let ref = db.collection("users").document(uid).collection("inventory")
-
-        let chunks = stride(from: 0, to: cards.count, by: 400).map { Array(cards[$0..<min($0 + 400, cards.count)]) }
-        for chunk in chunks {
-            let batch = db.batch()
-            for card in chunk {
-                let data: [String: Any] = [
-                    "val": card.currentValuation ?? 0.0,
-                    "curr": card.currencyCode ?? "USD",
-                    "rarity": card.rarity ?? "",
-                    "colors": card.colorIdentity ?? []
-                ]
-                batch.setData(data, forDocument: ref.document(card.id.uuidString), merge: true)
-            }
-            do {
-                try await batch.commit()
-            } catch { print("Batch Update Error: \(error)") }
-        }
-    }
 }