Add household sharing feature

✨ Features:
- Create household with 6-character shareable code
- Join existing household with code
- View household members and owner
- Leave household functionality
- Automatic inventory filtering by household
- Persistent household settings across app updates

🔧 Technical changes:
- Added Household model with Hive adapter (typeId: 4)
- Updated AppSettings with userName and currentHouseholdId fields
- Modified InventoryRepository to filter items by household
- Updated Add Item screen to set householdId on new items
- Added HouseholdScreen with full CRUD operations
- Integrated household sharing into Settings navigation

🎯 Behavior:
- Users not in a household see only their personal items
- Users in a household see shared household items
- New items automatically tagged with current household

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

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

lib/features/settings/models/household.dart

@@ -0,0 +1,45 @@
+import 'package:hive/hive.dart';
+
+part 'household.g.dart';
+
+@HiveType(typeId: 4)
+class Household extends HiveObject {
+  @HiveField(0)
+  late String id; // Unique household code
+
+  @HiveField(1)
+  late String name;
+
+  @HiveField(2)
+  late String ownerName; // Person who created the household
+
+  @HiveField(3)
+  late DateTime createdAt;
+
+  @HiveField(4)
+  List<String> members; // List of member names
+
+  Household({
+    required this.id,
+    required this.name,
+    required this.ownerName,
+    DateTime? createdAt,
+    List<String>? members,
+  })  : createdAt = createdAt ?? DateTime.now(),
+        members = members ?? [];
+
+  /// Generate a random household code
+  static String generateCode() {
+    const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
+    final random = DateTime.now().millisecondsSinceEpoch;
+    var code = '';
+    var seed = random;
+
+    for (var i = 0; i < 6; i++) {
+      seed = (seed * 1103515245 + 12345) & 0x7fffffff;
+      code += chars[seed % chars.length];
+    }
+
+    return code;
+  }
+}