Add real-time inventory syncing across devices v1.1.0

🔄 Inventory Sync Features:
- Automatic sync to Firebase when adding/updating/deleting items
- Real-time listener pulls changes from other devices
- Bi-directional sync keeps all household devices in sync
- Conflict resolution based on lastModified timestamp
- Firebase version always wins on conflicts

📱 How It Works:
Device A adds item → syncs to Firebase → Device B receives update
Device B updates item → syncs to Firebase → Device A receives update
Device A deletes item → syncs to Firebase → Device B removes item

🔧 Technical Implementation:
- InventorySyncService: Real-time Firestore listener
- Repository hooks: add/update/delete sync to Firebase
- HomeScreen lifecycle: starts/stops sync automatically
- Conflict resolution: newer timestamp wins
- Local Hive + Cloud Firestore hybrid architecture

📁 New Files:
- lib/features/household/services/inventory_sync_service.dart

✨ Updated Files:
- lib/features/inventory/repositories/inventory_repository_impl.dart
  - Added Firebase sync on add/update/delete operations
  - Maintains local Hive for offline access
- lib/features/home/screens/home_screen.dart
  - Starts sync service on init if in household
  - Stops sync service on dispose

⚠️ Requirements:
- Firebase must be configured (see FIREBASE_SETUP.md)
- Internet connection required for cross-device sync
- Local Hive works offline, syncs when online

✅ Build Status:
- APK: 63.4MB
- Package: com.github.mystiatech.sage
- Version: 1.1.0+2

🎯 Next Steps for User:
1. Set up Firebase (FIREBASE_SETUP.md)
2. Replace google-services.json with real file
3. Rebuild APK
4. Install on both devices
5. Create/join household
6. Add items → they sync! 🎉

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/features/household/services/inventory_sync_service.dart

@@ -0,0 +1,102 @@
+import 'dart:async';
+import 'package:cloud_firestore/cloud_firestore.dart';
+import '../../../data/local/hive_database.dart';
+import '../../inventory/models/food_item.dart';
+
+/// Service for syncing inventory items with Firebase in real-time
+class InventorySyncService {
+  final FirebaseFirestore _firestore = FirebaseFirestore.instance;
+  StreamSubscription? _itemsSubscription;
+
+  /// Start listening to household items from Firebase
+  Future<void> startSync(String householdId) async {
+    await stopSync(); // Stop any existing subscription
+
+    _itemsSubscription = _firestore
+        .collection('households')
+        .doc(householdId)
+        .collection('items')
+        .snapshots()
+        .listen((snapshot) async {
+      await _handleItemsUpdate(snapshot, householdId);
+    });
+  }
+
+  /// Stop listening to Firebase updates
+  Future<void> stopSync() async {
+    await _itemsSubscription?.cancel();
+    _itemsSubscription = null;
+  }
+
+  /// Handle updates from Firebase
+  Future<void> _handleItemsUpdate(
+    QuerySnapshot snapshot,
+    String householdId,
+  ) async {
+    final box = await HiveDatabase.getFoodBox();
+
+    // Track Firebase item IDs
+    final firebaseItemIds = <String>{};
+
+    for (final doc in snapshot.docs) {
+      firebaseItemIds.add(doc.id);
+      final data = doc.data() as Map<String, dynamic>;
+
+      // Check if item exists in local Hive
+      final itemKey = int.tryParse(doc.id);
+      if (itemKey != null) {
+        final existingItem = box.get(itemKey);
+
+        // Create or update item
+        final item = _createFoodItemFromData(data, householdId);
+
+        if (existingItem == null) {
+          // New item from Firebase - add to local Hive with specific key
+          await box.put(itemKey, item);
+        } else {
+          // Update existing item if Firebase version is newer
+          final firebaseModified = DateTime.parse(data['lastModified'] as String);
+          final localModified = existingItem.lastModified ?? DateTime(2000);
+
+          if (firebaseModified.isAfter(localModified)) {
+            // Firebase version is newer - update local
+            await box.put(itemKey, item);
+          }
+        }
+      }
+    }
+
+    // Delete items that no longer exist in Firebase
+    final itemsToDelete = <int>[];
+    for (final item in box.values) {
+      if (item.householdId == householdId && item.key != null) {
+        if (!firebaseItemIds.contains(item.key.toString())) {
+          itemsToDelete.add(item.key!);
+        }
+      }
+    }
+
+    for (final key in itemsToDelete) {
+      await box.delete(key);
+    }
+  }
+
+  /// Create FoodItem from Firebase data
+  FoodItem _createFoodItemFromData(Map<String, dynamic> data, String householdId) {
+    return FoodItem()
+      ..name = data['name'] as String
+      ..barcode = data['barcode'] as String?
+      ..quantity = data['quantity'] as int
+      ..unit = data['unit'] as String?
+      ..purchaseDate = DateTime.parse(data['purchaseDate'] as String)
+      ..expirationDate = DateTime.parse(data['expirationDate'] as String)
+      ..locationIndex = data['locationIndex'] as int
+      ..category = data['category'] as String?
+      ..photoUrl = data['photoUrl'] as String?
+      ..notes = data['notes'] as String?
+      ..userId = data['userId'] as String?
+      ..householdId = householdId
+      ..lastModified = DateTime.parse(data['lastModified'] as String)
+      ..syncedToCloud = true;
+  }
+}