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

lib/anonymous-lists.ts

@@ -0,0 +1,85 @@
+/**
+ * Utilities for managing anonymous list IDs in localStorage
+ * This helps track lists created before user registration for migration
+ * 
+ * Requirements: 10.2, 10.5 - Migration path for existing anonymous lists
+ */
+
+const STORAGE_KEY = 'anonymousListIds'
+
+/**
+ * Add a list ID to the anonymous lists tracking
+ */
+export function trackAnonymousList(listId: string): void {
+  if (typeof window === 'undefined') return
+
+  try {
+    const stored = localStorage.getItem(STORAGE_KEY)
+    const listIds: string[] = stored ? JSON.parse(stored) : []
+    
+    if (!listIds.includes(listId)) {
+      listIds.push(listId)
+      localStorage.setItem(STORAGE_KEY, JSON.stringify(listIds))
+    }
+  } catch (error) {
+    console.error('Error tracking anonymous list:', error)
+  }
+}
+
+/**
+ * Remove a list ID from anonymous lists tracking
+ */
+export function untrackAnonymousList(listId: string): void {
+  if (typeof window === 'undefined') return
+
+  try {
+    const stored = localStorage.getItem(STORAGE_KEY)
+    if (!stored) return
+
+    const listIds: string[] = JSON.parse(stored)
+    const filtered = listIds.filter(id => id !== listId)
+    
+    if (filtered.length > 0) {
+      localStorage.setItem(STORAGE_KEY, JSON.stringify(filtered))
+    } else {
+      localStorage.removeItem(STORAGE_KEY)
+    }
+  } catch (error) {
+    console.error('Error untracking anonymous list:', error)
+  }
+}
+
+/**
+ * Get all tracked anonymous list IDs
+ */
+export function getAnonymousListIds(): string[] {
+  if (typeof window === 'undefined') return []
+
+  try {
+    const stored = localStorage.getItem(STORAGE_KEY)
+    return stored ? JSON.parse(stored) : []
+  } catch (error) {
+    console.error('Error getting anonymous list IDs:', error)
+    return []
+  }
+}
+
+/**
+ * Clear all tracked anonymous list IDs
+ */
+export function clearAnonymousLists(): void {
+  if (typeof window === 'undefined') return
+
+  try {
+    localStorage.removeItem(STORAGE_KEY)
+  } catch (error) {
+    console.error('Error clearing anonymous lists:', error)
+  }
+}
+
+/**
+ * Check if there are any tracked anonymous lists
+ */
+export function hasAnonymousLists(): boolean {
+  return getAnonymousListIds().length > 0
+}