feat: implement real-time list synchronization

- Add useRealtimeSync hook for polling server updates every 3 seconds
- Create API endpoint to check last updated timestamp for lists
- Integrate real-time sync into ListDetailView component
- Automatically refresh when other users make changes to shared lists
- Poll on interval and when page becomes visible again

app/api/lists/[id]/last-updated/route.ts

@@ -0,0 +1,82 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { prisma } from '@/lib/db'
+import { auth } from '@/auth'
+
+export const dynamic = 'force-dynamic'
+export const revalidate = 0
+
+export async function GET(
+  request: NextRequest,
+  { params }: { params: { id: string } }
+) {
+  try {
+    const session = await auth()
+    if (!session?.user?.id) {
+      return NextResponse.json(
+        { error: 'Unauthorized' },
+        { status: 401 }
+      )
+    }
+
+    const userId = session.user.id
+    const listId = params.id
+
+    // Check if user has access to this list
+    const list = await prisma.list.findFirst({
+      where: {
+        id: listId,
+        OR: [
+          { ownerId: userId },
+          {
+            collaborators: {
+              some: {
+                userId: userId,
+              },
+            },
+          },
+        ],
+      },
+      select: {
+        updatedAt: true,
+        items: {
+          select: {
+            updatedAt: true,
+          },
+          orderBy: {
+            updatedAt: 'desc',
+          },
+          take: 1,
+        },
+      },
+    })
+
+    if (!list) {
+      return NextResponse.json(
+        { error: 'List not found or access denied' },
+        { status: 404 }
+      )
+    }
+
+    // Get the most recent update time (either list or its items)
+    const listUpdated = list.updatedAt.getTime()
+    const itemUpdated = list.items[0]?.updatedAt.getTime() || 0
+    const lastUpdated = Math.max(listUpdated, itemUpdated)
+
+    return NextResponse.json(
+      { lastUpdated: new Date(lastUpdated).toISOString() },
+      {
+        headers: {
+          'Cache-Control': 'no-store, no-cache, must-revalidate',
+          'Pragma': 'no-cache',
+          'Expires': '0',
+        },
+      }
+    )
+  } catch (error) {
+    console.error('Error fetching last updated timestamp:', error)
+    return NextResponse.json(
+      { error: 'Internal server error' },
+      { status: 500 }
+    )
+  }
+}