Implement cross-device prayer synchronization

- Replace localStorage with Firestore for prayer persistence
- Prayers now sync across all devices when logged in
- Add userId to prayers for proper user association
- Update archive/unarchive to work with Firestore
- Update Firestore rules for user-specific prayer access
- Remove localStorage dependencies

firestore.rules

@@ -6,9 +6,12 @@ service cloud.firestore {
       allow read, write: if request.auth != null && request.auth.uid == userId;
     }
 
-    // Allow authenticated users to read/write prayers
+    // Allow authenticated users to read/write their own prayers
     match /prayers/{prayerId} {
-      allow read, write: if request.auth != null;
+      allow read, write: if request.auth != null && 
+        resource.data.userId == request.auth.uid;
+      allow create: if request.auth != null && 
+        request.resource.data.userId == request.auth.uid;
     }
 
     // Allow server-side API access for scheduled emails (no auth required for GET)