feat: Implement Supabase authentication, account management UI, and entitlement services.

frontend/app/auth/callback/route.ts

@@ -0,0 +1,32 @@
+import { NextRequest, NextResponse } from "next/server";
+import { createSupabaseRouteClient, hasSupabaseServerEnv } from "@/lib/supabase/server";
+
+function normalizeNextPath(input: string | null) {
+  const fallback = "/";
+  const raw = String(input || "").trim();
+  if (!raw) return fallback;
+  if (!raw.startsWith("/")) return fallback;
+  if (raw.startsWith("//")) return fallback;
+  return raw;
+}
+
+export async function GET(request: NextRequest) {
+  const nextPath = normalizeNextPath(request.nextUrl.searchParams.get("next"));
+  const redirectUrl = request.nextUrl.clone();
+  redirectUrl.pathname = nextPath;
+  redirectUrl.search = "";
+
+  if (!hasSupabaseServerEnv()) {
+    return NextResponse.redirect(redirectUrl);
+  }
+
+  const response = NextResponse.redirect(redirectUrl);
+  const supabase = createSupabaseRouteClient(request, response);
+  const code = request.nextUrl.searchParams.get("code");
+  if (code) {
+    await supabase.auth.exchangeCodeForSession(code);
+  }
+
+  return response;
+}
+