migrate auth from next-auth to better-auth with magic link support

Replace next-auth with better-auth, adding magic link email login as
the primary auth method and GitHub OAuth as an optional social provider.

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

components/user.tsx

@@ -1,10 +1,10 @@
-import { useSession, signOut } from 'next-auth/react'
+import { useSession, signOut } from '../lib/auth-client'
 import Link from 'next/link'
 
 export default function User() {
-  const { data, status } = useSession()
+  const { data: session, isPending } = useSession()
 
-  if (status === 'loading' || !data?.user?.email) {
+  if (isPending || !session?.user?.email) {
     return null
   }
 
@@ -17,7 +17,7 @@ export default function User() {
         Admin
       </Link>
       <div className="font-extrabold bg-red-400 px-2 py-1 mr-3 rounded-xs">
-        {data.user.email}
+        {session.user.email}
       </div>
       <button onClick={() => signOut()} className="btn btn-blue">
         Logout