Fix admin-web build: drop invalid --webpack flag on Next.js 14

The admin app runs Next.js 14.2.18, where `next build --webpack` is an
unknown option (the flag only exists in Next 15+). This broke the CI
admin-web image build. Other web apps stay on the flag since they're on
Next 16.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/admin/package.json

@@ -4,7 +4,7 @@
   "private": true,
   "scripts": {
     "dev": "next dev -p 3102",
-    "build": "next build --webpack",
+    "build": "next build",
     "start": "next start -p 3102",
     "lint": "next lint",
     "typecheck": "tsc --noEmit"