feat(security): add a report-only CSP and security headers

nginx.conf is now an nginx template, so connect-src takes
AUTH_HOST and API_HOST from the container env. The image
sets both to empty by default, so nginx still starts
without them. Report-only until production shows no
violation. Also nosniff, Referrer-Policy and
Permissions-Policy.

nginx.conf

@@ -2,6 +2,15 @@ server {
     listen 80;
     root /usr/share/nginx/html;
 
+    # An nginx template: AUTH_HOST and API_HOST come from the container env at start (see Dockerfile).
+    # Report-Only until production shows no violation in the browser console; then rename the header.
+    add_header Content-Security-Policy-Report-Only "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https://*.steamstatic.com https://steamcdn-a.akamaihd.net https://media.steampowered.com https://images.igdb.com; font-src 'self' data:; connect-src 'self' https://${AUTH_HOST} https://${API_HOST}; manifest-src 'self'; object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+    add_header Permissions-Policy "camera=(), microphone=(), geolocation=()" always;
+    # Keeps the headers above in the locations that add their own Cache-Control (nginx 1.29.3 and later).
+    add_header_inherit merge;
+
     # Vite puts a content hash in these file names, so they never change.
     location /assets/ {
         add_header Cache-Control "public, max-age=31536000, immutable";