feat: implement message CRUD and dark mode

- Add Message, Recipient models and migration
- Create message API routes (GET, POST, PATCH, DELETE)
- Fix IDOR vulnerability in PATCH (verify ownership before delete)
- Add dashboard and message pages (list, new, detail)
- Add UI components (card, input, label, textarea)
- Enable dark mode in layout
- Add session user id typing to NextAuth
- Add AGENTS.md memory bank documentation

prisma/schema.prisma

@@ -1,4 +1,3 @@
-// ... existing code ...
 generator client {
   provider = "prisma-client-js"
 }
@@ -42,6 +41,37 @@ model User {
   image         String?
   accounts      Account[]
   sessions      Session[]
+  messages      Message[]
+}
+
+model Message {
+  id          String       @id @default(cuid())
+  userId      String
+  title       String
+  content     String?      @db.Text
+  status      String       @default("DRAFT")
+  lastPing    DateTime     @default(now())
+  checkInterval Int        @default(24)
+  user        User         @relation(fields: [userId], references: [id], onDelete: Cascade)
+  recipients  Recipient[]
+  attachments Attachment[]
+  createdAt   DateTime     @default(now())
+  updatedAt   DateTime     @updatedAt
+}
+
+model Recipient {
+  id        String  @id @default(cuid())
+  messageId String
+  email     String
+  message   Message @relation(fields: [messageId], references: [id], onDelete: Cascade)
+}
+
+model Attachment {
+  id        String  @id @default(cuid())
+  messageId String
+  fileUrl   String
+  fileName  String
+  message   Message @relation(fields: [messageId], references: [id], onDelete: Cascade)
 }
 
 model VerificationToken {