Add OAuth callback route handler for Supabase auth

frontend-next/src/app/auth/callback/route.ts

@@ -0,0 +1,45 @@
+import { createClient } from "@supabase/supabase-js";
+import { NextRequest, NextResponse } from "next/server";
+
+export async function GET(request: NextRequest) {
+  const { searchParams, origin } = new URL(request.url);
+
+  const code = searchParams.get("code");
+  const error = searchParams.get("error");
+  const errorDescription = searchParams.get("error_description");
+
+  // GitHub (or Supabase) returned an OAuth error
+  if (error) {
+    const params = new URLSearchParams({ error: errorDescription ?? error });
+    return NextResponse.redirect(`${origin}/?${params.toString()}`);
+  }
+
+  // No code present — nothing to exchange
+  if (!code) {
+    return NextResponse.redirect(
+      `${origin}/?error=Missing+authorization+code`
+    );
+  }
+
+  const supabaseUrl = process.env.NEXT_PUBLIC_SUPABASE_URL;
+  const supabaseKey =
+    process.env.SUPABASE_KEY ?? process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY;
+
+  if (!supabaseUrl || !supabaseKey) {
+    return NextResponse.redirect(
+      `${origin}/?error=Auth+service+not+configured`
+    );
+  }
+
+  const supabase = createClient(supabaseUrl, supabaseKey);
+
+  const { error: exchangeError } = await supabase.auth.exchangeCodeForSession(code);
+
+  if (exchangeError) {
+    const params = new URLSearchParams({ error: exchangeError.message });
+    return NextResponse.redirect(`${origin}/?${params.toString()}`);
+  }
+
+  // Successful exchange — send the user to the dashboard
+  return NextResponse.redirect(`${origin}/`);
+}