feat: implement complete web push notification system

- Add self-hosted push notifications using VAPID (no external services)
- Create PushSubscription model in database
- Implement server-side push logic with web-push library
- Add notification helpers for list events (items, invitations, collaborators)
- Create NotificationSettings UI component for profile page
- Add usePushNotifications React hook for subscription management
- Create API routes for subscription management and VAPID key
- Update service worker with push event handlers
- Integrate notifications into item actions (add, remove, complete)
- Integrate notifications into sharing actions (invite, accept, remove)
- Add VAPID key generation script
- Add comprehensive documentation and setup guides

Notifications are sent when:
- Items are added/removed/completed in shared lists
- Users are invited to lists
- Invitations are accepted
- Collaborators are removed from lists

Works on iOS 16.4+ (when installed to home screen), Android, and desktop browsers.

lib/actions.ts

@@ -12,6 +12,11 @@ import {
   validateTags,
   sanitizeString,
 } from './validation'
+import {
+  notifyItemAdded,
+  notifyItemRemoved,
+  notifyItemCompleted,
+} from './notification-helpers'
 
 // Retry utility for database operations
 async function retryOperation<T>(
@@ -370,6 +375,14 @@ export async function createItem(
       })
     )
     
+    // Send notification to collaborators
+    try {
+      await notifyItemAdded(listId, item.title, userId)
+    } catch (error) {
+      console.error('Failed to send notification:', error)
+      // Don't fail the action if notification fails
+    }
+    
     revalidatePath(`/list/${listId}`)
     return item
   } catch (error) {
@@ -437,6 +450,15 @@ export async function toggleItem(itemId: string) {
       })
     )
     
+    // Send notification when item is completed (not uncompleted)
+    if (updated.completed) {
+      try {
+        await notifyItemCompleted(item.listId, updated.title, userId)
+      } catch (error) {
+        console.error('Failed to send notification:', error)
+      }
+    }
+    
     revalidatePath(`/list/${item.listId}`)
     return updated
   } catch (error) {
@@ -609,6 +631,13 @@ export async function deleteItem(itemId: string) {
       })
     )
     
+    // Send notification to collaborators
+    try {
+      await notifyItemRemoved(item.listId, item.title, userId)
+    } catch (error) {
+      console.error('Failed to send notification:', error)
+    }
+    
     revalidatePath(`/list/${item.listId}`)
     return { success: true }
   } catch (error) {