Fix nginx serving no static assets in production compose (ported from qr-vip)

Same bug as qr-vip (2026-07-11): nginx and php-fpm are separate
containers in docker-compose.prod.yml, but nginx had no copy of the
static assets it needs to serve directly, so they'd fall through
try_files to the login-gated index.php. Not currently visible on
qr.ensembia.com (which runs docker-compose.yml, the single-container dev
variant, not this prod file) but would hit anyone using the production
compose as documented. New Dockerfile.nginx copies in dist/, plugins/,
manifest.json, service-worker.js, and favicon.ico.

docker-compose.prod.yml

@@ -1,6 +1,8 @@
 services:
   nginx:
-    image: "nginx:1.27-alpine"
+    build:
+      context: .
+      dockerfile: Dockerfile.nginx
     restart: "unless-stopped"
     ports:
       - "80:80"