chore: migrate middleware to proxy as per Next.js 2026 convention

proxy.ts

@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { decrypt } from '@/app/lib/auth';
 
-export async function middleware(request: NextRequest) {
+export async function proxy(request: NextRequest) {
   const session = request.cookies.get('session')?.value;
   const { pathname } = request.nextUrl;