Implement storage architecture from ai_blueprint.md

Primary sync: replace PersistenceActor JSON file with SwiftData + CloudKit
- Add SavedCardModel (@Model class) and PersistenceController (ModelContainer
  with .automatic CloudKit, fallback to local). BackgroundPersistenceActor
  (@ModelActor) handles all DB I/O off the main thread.
- One-time migration imports user_collection.json into SwiftData and renames
  the original file to prevent re-import.
- Inject modelContainer into SwiftUI environment in IYmtgApp.

Image storage: Documents/UserContent/ subfolder (blueprint requirement)
- ImageManager.dir now targets iCloud Documents/UserContent/ (or local equiv).
- migrateImagesToUserContent() moves existing JPGs to the new subfolder on
  first launch; called during the SwiftData migration.

Firebase: demoted to optional manual backup (metadata only, no images)
- Remove all automatic CloudEngine.save/delete/batchUpdatePrices calls from
  CollectionViewModel mutations.
- Add backupAllToFirebase() for user-triggered metadata sync.
- Add isFirebaseBackupEnabled to AppConfig (default false).
- Add Cloud Backup section in Library settings with iCloud vs Firebase
  explanation and "Backup Metadata to Firebase Now" button.

Also: full modular refactor (Data/, Features/, Services/ directories) and
README updated with CloudKit setup steps and revised release checklist.

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

IYmtg_App_iOS/AppConfig.swift

@@ -44,6 +44,13 @@ struct AppConfig {
         get { UserDefaults.standard.bool(forKey: "TrainingOptIn") }
         set { UserDefaults.standard.set(newValue, forKey: "TrainingOptIn") }
     }
+
+    // Firebase secondary backup — metadata only (no card images).
+    // Default: false. Users opt-in via the Cloud Backup section in Library settings.
+    static var isFirebaseBackupEnabled: Bool {
+        get { UserDefaults.standard.bool(forKey: "FirebaseBackupEnabled") }
+        set { UserDefaults.standard.set(newValue, forKey: "FirebaseBackupEnabled") }
+    }
     
     static var scryfallUserAgent: String {
         return "IYmtg/1.0 (\(contactEmail))"
@@ -54,7 +61,9 @@ struct AppConfig {
         if contactEmail.contains("yourdomain.com") {
             fatalError("🛑 SETUP ERROR: Change 'contactEmail' in AppConfig.swift to your real email.")
         }
-        if let first = tipJarProductIDs.first, (first.contains("yourname") || first == "com.iymtg.app.tip") {
+        if tipJarProductIDs.isEmpty {
+            print("⚠️ CONFIG WARNING: 'tipJarProductIDs' is empty. Tip Jar will not be available.")
+        } else if let first = tipJarProductIDs.first, (first.contains("yourname") || first == "com.iymtg.app.tip") {
             print("⚠️ CONFIG WARNING: 'tipJarProductIDs' contains placeholder. IAP will not load.")
         }
         #endif