Add phone and birthday fields to User model with admin editor

- Add phone/birthday columns via Prisma migration
- Admin user page shows phone and birthday with inline edit UI
- UserContactEditor client component for editing contact info
- updateUserContact server action with admin-only guard

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -18,6 +18,8 @@ model User {
   emailVerified Boolean   @default(false)
   image         String?
   role          String    @default("student") // student | curator | admin
+  phone         String?
+  birthday      DateTime?
   banned        Boolean?  @default(false)
   banReason     String?
   banExpires    DateTime?