fix: remove @supabase/ssr entirely to fix MIDDLEWARE_INVOCATION_FAILED

@supabase/ssr uses __dirname internally which crashes in Vercel's Edge
runtime, causing MIDDLEWARE_INVOCATION_FAILED even without a middleware
file. Replaced with @supabase/supabase-js directly:
- Server client: manual cookie-based session restoration
- Browser client: direct createClient from supabase-js
- Admin client: already using supabase-js

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

next.config.ts

@@ -1,7 +1,5 @@
 import type { NextConfig } from "next";
 
-const nextConfig: NextConfig = {
-  serverExternalPackages: ["@supabase/ssr"],
-};
+const nextConfig: NextConfig = {};
 
 export default nextConfig;