fix: mobile configurator panel flows in-page so Proceed button is reachable

On mobile the configurator was a 100vh frame with an inner scrolling
bottom-sheet embedded inside the scrolling /configure page. Page scroll
hijacked the gesture and skipped the frame, trapping the Proceed to Order
and Reset buttons in the inner scroll. Drop the nested scroll on <=768px:
canvas stays fixed on top, the full config panel flows in normal page
scroll so the CTAs are always visible. Landscape side-by-side preserved.

Also pin outputFileTracingRoot to the project dir to silence the
multiple-lockfile workspace-root warning.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

next.config.mjs

@@ -1,6 +1,12 @@
+import { fileURLToPath } from 'node:url';
+import { dirname } from 'node:path';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   output: 'standalone',
+  outputFileTracingRoot: __dirname,
   images: {
     unoptimized: true,
   },