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>

helpers/url.ts

@@ -10,4 +10,4 @@ if (process.env.SITE_URL) {
 
 export function getBaseURL(): string {
   return URL
-}
+}

helpers/withAuth.tsx

@@ -1,17 +1,14 @@
-import { SessionProvider } from 'next-auth/react'
 import Auth from '../components/auth'
 
 // This is the HOC
 function withAuth(WrappedComponent: React.FunctionComponent) {
   // Return a new component
-  function withAuth({ session, ...pageProps }) {
+  function withAuth(pageProps) {
     // Render the WrappedComponent with additional props
     return (
-      <SessionProvider session={session}>
-        <Auth>
-          <WrappedComponent {...pageProps} />
-        </Auth>
-      </SessionProvider>
+      <Auth>
+        <WrappedComponent {...pageProps} />
+      </Auth>
     )
   }