🐛 fix: SPA HTML entry returns stale content after server upgrade (#12998)

fix: add no-cache header to SPA HTML entry point

Prevent stale SPA HTML from being served after server upgrades.
JS/CSS assets still cache normally via hashed filenames.

src/app/spa/[variants]/[[...path]]/route.ts

@@ -220,6 +220,7 @@ export async function GET(
 
   return new Response(html, {
     headers: {
+      'Cache-Control': 'no-cache',
       'content-type': 'text/html; charset=utf-8',
     },
   });