fix: split app into (main)/(sandbox) route groups so plugin iframe hydrates properly

app/(sandbox)/plugin-sandbox/page.tsx

@@ -0,0 +1,10 @@
+import { SandboxRuntime } from '@/lib/plugin-sandbox/runtime';
+
+// Must be dynamic so the per-request CSP nonce from proxy.ts is embedded in
+// Next's injected hydration/chunk scripts. With force-static, those scripts
+// render without a nonce and the strict sandbox CSP blocks them.
+export const dynamic = 'force-dynamic';
+
+export default function PluginSandboxPage() {
+  return <SandboxRuntime />;
+}