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
-}
+}