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.

prisma/schema.prisma

@@ -8,20 +8,35 @@ datasource db {
 }
 
 model User {
-  id                   String       @id @default(cuid())
-  email                String       @unique
+  id                   String             @id @default(cuid())
+  email                String             @unique
   name                 String?
   passwordHash         String
-  createdAt            DateTime     @default(now())
-  updatedAt            DateTime     @updatedAt
-  ownedLists           List[]       @relation("ListOwner")
+  createdAt            DateTime           @default(now())
+  updatedAt            DateTime           @updatedAt
+  ownedLists           List[]             @relation("ListOwner")
   collaborations       Collaborator[]
-  sentInvitations      Invitation[] @relation("InvitationInviter")
-  receivedInvitations  Invitation[] @relation("InvitationReceiver")
+  sentInvitations      Invitation[]       @relation("InvitationInviter")
+  receivedInvitations  Invitation[]       @relation("InvitationReceiver")
+  pushSubscriptions    PushSubscription[]
 
   @@index([email])
 }
 
+model PushSubscription {
+  id         String   @id @default(cuid())
+  userId     String
+  endpoint   String   @db.Text
+  p256dh     String   @db.Text
+  auth       String   @db.Text
+  createdAt  DateTime @default(now())
+  updatedAt  DateTime @updatedAt
+  user       User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([userId, endpoint(255)])
+  @@index([userId])
+}
+
 model List {
   id            String         @id @default(cuid())
   title         String