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/denied.tsx

@@ -1,4 +1,4 @@
-import { signIn } from 'next-auth/react'
+import { signIn } from '../lib/auth-client'
 
 export default function Denied() {
   return (
@@ -8,7 +8,12 @@ export default function Denied() {
         <a
           onClick={(e) => {
             e.preventDefault()
-            signIn()
+            if (process.env.NEXT_PUBLIC_GITHUB_ENABLED) {
+              signIn.social({
+                provider: "github",
+                callbackURL: window.location.href,
+              })
+            }
           }}
         >
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