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>

FIREBASE_SETUP.md

@@ -0,0 +1,120 @@
+# Firebase Setup Guide for Sage
+
+## Step 1: Create Firebase Project
+
+1. Go to [Firebase Console](https://console.firebase.google.com/)
+2. Click "Add project"
+3. Enter project name: `sage-kitchen-management`
+4. Disable Google Analytics (optional for this app)
+5. Click "Create project"
+
+## Step 2: Add Android App
+
+1. In Firebase Console, click the Android icon to add an Android app
+2. Enter package name: `com.example.sage` (must match AndroidManifest.xml)
+3. App nickname: `Sage` (optional)
+4. Debug signing certificate SHA-1: (optional, skip for now)
+5. Click "Register app"
+
+## Step 3: Download Configuration File
+
+1. Download the `google-services.json` file
+2. Place it in: `android/app/google-services.json`
+
+## Step 4: Set up Firestore Database
+
+1. In Firebase Console, go to "Build" → "Firestore Database"
+2. Click "Create database"
+3. Choose "Start in test mode" for development
+4. Select a Firestore location (e.g., `us-central`)
+5. Click "Enable"
+
+### Security Rules (update after testing)
+
+For development/testing, use test mode rules:
+```
+rules_version = '2';
+service cloud.firestore {
+  match /databases/{database}/documents {
+    match /{document=**} {
+      allow read, write: if request.time < timestamp.date(2025, 12, 31);
+    }
+  }
+}
+```
+
+For production, update to:
+```
+rules_version = '2';
+service cloud.firestore {
+  match /databases/{database}/documents {
+    // Allow anyone to read household data by code
+    match /households/{householdId} {
+      allow read: if true;
+      allow create: if true;
+      allow update: if true;
+      allow delete: if request.auth != null;
+
+      // Allow household members to manage items
+      match /items/{itemId} {
+        allow read: if true;
+        allow write: if true;
+      }
+    }
+  }
+}
+```
+
+## Step 5: Update Android Build Files (Already Done)
+
+The following files need to be updated (will be done automatically):
+
+1. `android/build.gradle` - Add Google Services plugin
+2. `android/app/build.gradle` - Apply Google Services plugin
+
+## Step 6: Initialize Firebase in App
+
+The app will automatically initialize Firebase on startup.
+
+## Firestore Data Structure
+
+```
+households (collection)
+  └── {householdCode} (document)
+      ├── id: string
+      ├── name: string
+      ├── ownerName: string
+      ├── createdAt: string (ISO 8601)
+      └── members: array<string>
+      └── items (subcollection)
+          └── {itemKey} (document)
+              ├── name: string
+              ├── barcode: string?
+              ├── quantity: number
+              ├── unit: string?
+              ├── purchaseDate: string (ISO 8601)
+              ├── expirationDate: string (ISO 8601)
+              ├── locationIndex: number
+              ├── category: string?
+              ├── photoUrl: string?
+              ├── notes: string?
+              ├── userId: string?
+              ├── householdId: string
+              ├── lastModified: string (ISO 8601)
+              └── syncedToCloud: boolean
+```
+
+## Testing
+
+1. Create a household on Device A
+2. Note the 6-character code
+3. Join the household from Device B using the code
+4. Add items on Device A → should appear on Device B
+5. Add items on Device B → should appear on Device A
+
+## Troubleshooting
+
+- **"google-services.json not found"**: Make sure file is in `android/app/` directory
+- **Build errors**: Run `flutter clean && flutter pub get`
+- **Permission denied**: Check Firestore security rules in Firebase Console
+- **Items not syncing**: Check internet connection and Firebase Console logs