fix: resolve __dirname error by forcing Node.js runtime on all server routes

- Add serverExternalPackages for @supabase/ssr in next.config.ts
- Add export const runtime = 'nodejs' to all pages/routes using Supabase
- Replace createAdminClient to use @supabase/supabase-js directly (no SSR)
- Prevents @supabase/ssr from running in Edge runtime on Vercel

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

app/api/stripe/create-checkout/route.ts

@@ -2,6 +2,8 @@ import { NextResponse } from "next/server";
 import { stripe } from "@/lib/stripe/client";
 import { getBaseUrl } from "@/lib/utils";
 
+export const runtime = "nodejs";
+
 export async function POST(request: Request) {
   try {
     const body = await request.json();