Add Firebase cloud sync for household sharing v1.1.0

✨ New Features:
- Firebase Firestore integration for real-time household sharing
- Create household → generates code stored in cloud
- Join household → looks up code from Firebase across devices
- Leave household → updates member list in cloud
- Cloud-first with local Hive fallback for offline

🔧 Technical Implementation:
- Added firebase_core and cloud_firestore dependencies
- Created FirebaseHouseholdService for all cloud operations
- Updated HouseholdScreen to use Firebase for create/join/leave
- Modified gradle files to support Google Services plugin
- Increased minSdk to 21 for Firebase compatibility
- Version bumped to 1.1.0+2 (MAJOR.MINOR.BUGFIX)

📁 New Files:
- lib/features/household/services/firebase_household_service.dart
- FIREBASE_SETUP.md - Complete setup instructions
- android/app/google-services.json - Placeholder (needs replacement)
- android/app/README_FIREBASE.md - Firebase config reminder

⚙️ Setup Required:
1. Create Firebase project at console.firebase.google.com
2. Add Android app with package name: com.sage.sage
3. Download real google-services.json
4. Enable Firestore Database in test mode
5. See FIREBASE_SETUP.md for complete instructions

🎯 How it works:
- Device A creates household → stored in Firestore
- Device B joins with code → reads from Firestore
- Both devices now share same household ID
- Inventory items sync via shared household ID

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

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

pubspec.lock

@@ -9,6 +9,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "61.0.0"
+  _flutterfire_internals:
+    dependency: transitive
+    description:
+      name: _flutterfire_internals
+      sha256: ff0a84a2734d9e1089f8aedd5c0af0061b82fb94e95260d943404e0ef2134b11
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.3.59"
   analyzer:
     dependency: transitive
     description:
@@ -145,6 +153,30 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.1.2"
+  cloud_firestore:
+    dependency: "direct main"
+    description:
+      name: cloud_firestore
+      sha256: "2d33da4465bdb81b6685c41b535895065adcb16261beb398f5f3bbc623979e9c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.6.12"
+  cloud_firestore_platform_interface:
+    dependency: transitive
+    description:
+      name: cloud_firestore_platform_interface
+      sha256: "413c4e01895cf9cb3de36fa5c219479e06cd4722876274ace5dfc9f13ab2e39b"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.6.12"
+  cloud_firestore_web:
+    dependency: transitive
+    description:
+      name: cloud_firestore_web
+      sha256: c1e30fc4a0fcedb08723fb4b1f12ee4e56d937cbf9deae1bda43cbb6367bb4cf
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.4.12"
   code_builder:
     dependency: transitive
     description:
@@ -217,6 +249,30 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "7.0.1"
+  firebase_core:
+    dependency: "direct main"
+    description:
+      name: firebase_core
+      sha256: "7be63a3f841fc9663342f7f3a011a42aef6a61066943c90b1c434d79d5c995c5"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.15.2"
+  firebase_core_platform_interface:
+    dependency: transitive
+    description:
+      name: firebase_core_platform_interface
+      sha256: "5873a370f0d232918e23a5a6137dbe4c2c47cf017301f4ea02d9d636e52f60f0"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.0.1"
+  firebase_core_web:
+    dependency: transitive
+    description:
+      name: firebase_core_web
+      sha256: "0ed0dc292e8f9ac50992e2394e9d336a0275b6ae400d64163fdf0a8a8b556c37"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.24.1"
   fixnum:
     dependency: transitive
     description: