fix: add nginx SPA fallback for BrowserRouter routing

Root cause: Docker container was 4 days old and nginx had no
try_files fallback for SPA routes like /details.

Changes:
- Add nginx.conf with try_files $uri $uri/ /index.html
- Update Dockerfile to copy nginx config
- Add production browser QA script

Production verified:
- https://arhipovdan.ru/ shows full-screen 3D demo
- https://arhipovdan.ru/details shows detailed page

nginx.conf

@@ -0,0 +1,20 @@
+server {
+    listen 80;
+    server_name localhost;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    location /assets/ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+
+    location /models/ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+}