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

components/RegisterForm.tsx

@@ -5,6 +5,7 @@ import { useRouter } from 'next/navigation'
 import { validateEmail, validatePassword, sanitizeString } from '@/lib/validation'
 import { registerUser } from '@/lib/auth-actions'
 import { useToast } from './ToastContainer'
+import ClaimListsModal from './ClaimListsModal'
 
 interface RegisterFormProps {
   callbackUrl?: string
@@ -18,6 +19,7 @@ export default function RegisterForm({ callbackUrl = '/' }: RegisterFormProps) {
   const [password, setPassword] = useState('')
   const [confirmPassword, setConfirmPassword] = useState('')
   const [isSubmitting, setIsSubmitting] = useState(false)
+  const [showClaimModal, setShowClaimModal] = useState(false)
   const [errors, setErrors] = useState<{
     name?: string
     email?: string
@@ -154,8 +156,19 @@ export default function RegisterForm({ callbackUrl = '/' }: RegisterFormProps) {
         }
       } else {
         showToast('Registration successful! Logging you in...', 'success')
-        router.push(callbackUrl)
-        router.refresh()
+        
+        // Check if there are lists to claim from localStorage
+        // Requirement 10.2 - Add UI for users to claim existing lists on first registration
+        const hasAnonymousLists = checkForAnonymousLists()
+        
+        if (hasAnonymousLists) {
+          // Show claim modal
+          setShowClaimModal(true)
+        } else {
+          // No lists to claim, proceed to app
+          router.push(callbackUrl)
+          router.refresh()
+        }
       }
     } catch (err) {
       console.error('Registration error:', err)
@@ -165,8 +178,42 @@ export default function RegisterForm({ callbackUrl = '/' }: RegisterFormProps) {
     }
   }
 
+  const checkForAnonymousLists = (): boolean => {
+    try {
+      const stored = localStorage.getItem('anonymousListIds')
+      if (stored) {
+        const listIds = JSON.parse(stored)
+        return Array.isArray(listIds) && listIds.length > 0
+      }
+      return false
+    } catch (err) {
+      console.error('Error checking for anonymous lists:', err)
+      return false
+    }
+  }
+
+  const handleClaimComplete = () => {
+    setShowClaimModal(false)
+    showToast('Lists claimed successfully!', 'success')
+    router.push(callbackUrl)
+    router.refresh()
+  }
+
+  const handleClaimClose = () => {
+    setShowClaimModal(false)
+    router.push(callbackUrl)
+    router.refresh()
+  }
+
   return (
-    <form onSubmit={handleSubmit} className="space-y-4 sm:space-y-5">
+    <>
+      <ClaimListsModal
+        isOpen={showClaimModal}
+        onClose={handleClaimClose}
+        onComplete={handleClaimComplete}
+      />
+      
+      <form onSubmit={handleSubmit} className="space-y-4 sm:space-y-5">
       <div>
         <label htmlFor="name" className="block text-sm font-medium text-gray-700 mb-2">
           Name *
@@ -296,5 +343,6 @@ export default function RegisterForm({ callbackUrl = '/' }: RegisterFormProps) {
         {isSubmitting ? 'Creating account...' : 'Create Account'}
       </button>
     </form>
+    </>
   )
 }