feat: implement data migration for existing lists

- Created migration-actions.ts with server actions for claiming lists
- Added ClaimListsModal component for UI during registration
- Integrated claim modal into RegisterForm
- Created anonymous-lists.ts utilities for tracking lists in localStorage
- Added migrate-lists.ts script for deployment migration
- Created comprehensive MIGRATION_GUIDE.md documentation
- Added migration tests with 100% pass rate
- Updated package.json with db:migrate script
- Added tsx as dev dependency for running migration scripts

Requirements: 10.2, 10.3, 10.4, 10.5
- Users can claim existing lists on first registration
- All list data is preserved during migration
- All lists have an owner after migration
- Migrated lists appear in user's account

MIGRATION_GUIDE.md

@@ -0,0 +1,160 @@
+# Data Migration Guide
+
+This guide explains how to migrate existing anonymous lists to the new user authentication system.
+
+## Overview
+
+The Ultimate Lists App now supports user authentication and private lists. This migration ensures that:
+- All existing lists are preserved (Requirement 10.3)
+- All lists have an associated owner (Requirement 10.4)
+- Users can claim their existing lists when they register (Requirement 10.2)
+- All list data including items, metadata, and timestamps remain unchanged (Requirement 10.3)
+
+## Migration Strategy
+
+### For New Deployments
+
+If you're deploying the authentication system for the first time:
+
+1. **Run the migration script** before enabling authentication:
+   ```bash
+   npx tsx scripts/migrate-lists.ts
+   ```
+
+   This script will:
+   - Create a system user if it doesn't exist
+   - Assign any orphaned lists to the system user
+   - Verify all lists have owners
+
+2. **Deploy the application** with authentication enabled
+
+3. **Users can claim lists** when they register:
+   - Lists created before registration are tracked in localStorage
+   - During registration, users are prompted to claim their lists
+   - Lists are transferred from system user to the new user account
+
+### For Existing Users
+
+When a user registers for the first time:
+
+1. The app checks localStorage for any tracked list IDs
+2. If lists are found, a modal appears asking the user to claim them
+3. User can select which lists to claim
+4. Selected lists are transferred to the user's account
+5. All list data (items, metadata, timestamps) is preserved
+
+## Technical Details
+
+### Database Schema Changes
+
+The migration adds:
+- `User` table with authentication fields
+- `ownerId` field on `List` table (required)
+- `Collaborator` table for list sharing
+- `Invitation` table for sharing invitations
+
+### Migration Script
+
+Location: `scripts/migrate-lists.ts`
+
+The script:
+- Creates a system user (`system@ultimatelists.app`)
+- Assigns orphaned lists to the system user
+- Verifies data integrity
+- Provides a migration summary
+
+### Client-Side Tracking
+
+Location: `lib/anonymous-lists.ts`
+
+Utilities for tracking anonymous lists:
+- `trackAnonymousList(listId)` - Add a list to tracking
+- `getAnonymousListIds()` - Get all tracked lists
+- `clearAnonymousLists()` - Clear tracking after migration
+- `hasAnonymousLists()` - Check if there are lists to migrate
+
+### Migration Actions
+
+Location: `lib/migration-actions.ts`
+
+Server actions for migration:
+- `getClaimableLists(listIds)` - Fetch lists that can be claimed
+- `claimLists(listIds)` - Transfer lists to user account
+- `hasExistingLists()` - Check if user has lists
+- `migrateOrphanedListsToSystemUser()` - One-time migration script
+
+### UI Components
+
+Location: `components/ClaimListsModal.tsx`
+
+Modal component that:
+- Displays claimable lists with details
+- Allows users to select which lists to claim
+- Handles the claim process
+- Clears localStorage after successful claim
+
+## Testing the Migration
+
+### Test Scenario 1: New User with Anonymous Lists
+
+1. Create lists without being logged in
+2. Register for an account
+3. Verify the claim modal appears
+4. Select lists to claim
+5. Verify lists appear in the user's account
+6. Verify all items and metadata are preserved
+
+### Test Scenario 2: New User without Anonymous Lists
+
+1. Register for an account without creating lists
+2. Verify no claim modal appears
+3. Verify user is redirected to the app
+
+### Test Scenario 3: Existing User
+
+1. Log in with an existing account
+2. Verify existing lists are displayed
+3. Verify all data is intact
+
+## Rollback Plan
+
+If issues occur during migration:
+
+1. **Database Rollback**:
+   ```bash
+   npx prisma migrate reset
+   ```
+
+2. **Restore from Backup**:
+   - Restore database from pre-migration backup
+   - Redeploy previous version of the application
+
+3. **Manual Data Recovery**:
+   - Export lists from system user
+   - Manually assign to correct users
+   - Verify data integrity
+
+## Monitoring
+
+After migration, monitor:
+- Number of orphaned lists (should be 0)
+- Number of lists owned by system user
+- User registration and claim rates
+- Any errors in migration logs
+
+## Support
+
+If users experience issues:
+1. Check browser console for errors
+2. Verify localStorage contains list IDs
+3. Check server logs for migration errors
+4. Manually transfer lists if needed using admin tools
+
+## Requirements Validation
+
+This migration satisfies:
+- ✓ Requirement 10.1: Authentication system provides migration path
+- ✓ Requirement 10.2: Users can claim existing lists on registration
+- ✓ Requirement 10.3: All list data is preserved during migration
+- ✓ Requirement 10.4: All lists have an associated owner
+- ✓ Requirement 10.5: Migrated lists appear in user's account