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>

lib/auth-client.ts

@@ -0,0 +1,8 @@
+import { createAuthClient } from "better-auth/react"
+import { magicLinkClient } from "better-auth/client/plugins"
+
+export const authClient = createAuthClient({
+  plugins: [magicLinkClient()],
+})
+
+export const { signIn, signOut, signUp, useSession } = authClient