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/sharing-actions.ts

@@ -6,6 +6,11 @@ import { getServerSession } from 'next-auth'
 import { authOptions } from '@/pages/api/auth/[...nextauth]'
 import { z } from 'zod'
 import { checkRateLimit, INVITATION_RATE_LIMIT } from './rate-limit'
+import {
+  notifyListInvitation,
+  notifyInvitationAccepted,
+  notifyRemovedFromList,
+} from './notification-helpers'
 
 // Validation schemas
 const inviteSchema = z.object({
@@ -163,6 +168,14 @@ export async function inviteUserToList(
       })
     )
     
+    // Send notification to the invitee
+    try {
+      await notifyListInvitation(invitation.id, validEmail)
+    } catch (error) {
+      console.error('Failed to send invitation notification:', error)
+      // Don't fail the action if notification fails
+    }
+    
     revalidatePath(`/list/${listId}`)
     return invitation
   } catch (error) {
@@ -320,6 +333,14 @@ export async function acceptInvitation(invitationId: string) {
       })
     )
     
+    // Notify the person who sent the invitation
+    try {
+      await notifyInvitationAccepted(invitationId)
+    } catch (error) {
+      console.error('Failed to send acceptance notification:', error)
+      // Don't fail the action if notification fails
+    }
+    
     revalidatePath('/')
     revalidatePath(`/list/${invitation.listId}`)
     return result
@@ -592,6 +613,20 @@ export async function removeCollaborator(
       throw new Error('List not found or you do not have permission to remove collaborators')
     }
     
+    // Get collaborator details before deleting
+    const collaborator = await retryOperation(() =>
+      prisma.collaborator.findUnique({
+        where: { id: collaboratorId },
+        include: {
+          user: {
+            select: {
+              id: true,
+            },
+          },
+        },
+      })
+    )
+    
     // Delete the collaborator
     await retryOperation(() =>
       prisma.collaborator.delete({
@@ -602,6 +637,16 @@ export async function removeCollaborator(
       })
     )
     
+    // Notify the removed user
+    if (collaborator) {
+      try {
+        await notifyRemovedFromList(collaborator.user.id, list.title, userId)
+      } catch (error) {
+        console.error('Failed to send removal notification:', error)
+        // Don't fail the action if notification fails
+      }
+    }
+    
     revalidatePath(`/list/${listId}`)
     return { success: true }
   } catch (error) {