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/auth.tsx

@@ -1,21 +1,42 @@
-import { useEffect } from 'react'
-
-import { useSession, signIn } from 'next-auth/react'
+import { useState } from 'react'
+import { useSession, signIn } from '../lib/auth-client'
+import Input from './input'
+import Button from './button'
 
 export default function Auth({ children }) {
-  const { data: session, status } = useSession()
+  const { data: session, isPending } = useSession()
   const isUser = !!session?.user
+  const [email, setEmail] = useState('')
+  const [loading, setLoading] = useState(false)
+  const [sent, setSent] = useState(false)
+  const [error, setError] = useState('')
 
-  useEffect(() => {
-    if (status === 'loading') return // Do nothing while loading
-    if (!isUser) signIn() // If not authenticated, force log in
-  }, [isUser, status])
+  if (isPending) return <div>Loading...</div>
 
-  if (isUser) {
-    return children
+  if (isUser) return children
+
+  if (process.env.NEXT_PUBLIC_GITHUB_ENABLED) {
+    signIn.social({ provider: "github", callbackURL: window.location.href })
+    return <div>Loading...</div>
   }
 
-  // Session is being fetched, or no user.
-  // If no user, useEffect() will redirect.
-  return <div>Loading...</div>
+  if (sent) return <div>E-Mail verschickt — bitte prüfe dein Postfach.</div>
+
+  async function handleSubmit(e: React.FormEvent) {
+    e.preventDefault()
+    setLoading(true)
+    setError('')
+    const result = await signIn.magicLink({ email, callbackURL: window.location.href })
+    if (result.error) setError(result.error.message)
+    else setSent(true)
+    setLoading(false)
+  }
+
+  return (
+    <form onSubmit={handleSubmit} className="max-w-sm mx-auto mt-16">
+      <Input label="E-Mail" name="email" type="email" value={email} onChange={e => setEmail(e.target.value)} required />
+      {error && <p className="text-red-500 text-sm mb-3">{error}</p>}
+      <Button type="submit" loading={loading}>Magic Link senden</Button>
+    </form>
+  )
 }