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

package.json

@@ -12,6 +12,7 @@
     "@auth/prisma-adapter": "^2.11.1",
     "@prisma/adapter-pg": "^7.2.0",
     "@prisma/client": "^7.2.0",
+    "@radix-ui/react-label": "^2.1.8",
     "@radix-ui/react-slot": "^1.2.4",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",