Recorded umpire voice, court staff, out-zone borders, intro overlay, no-cache deploys

- 23 pre-recorded umpire lines (macOS say, en-GB Daniel) played via WebAudio,
  works on mobile; speechSynthesis stays as fallback only
- umpire chair + two ball boys (generated sprites) dress the court
- court redrawn: doubles alleys, white perimeter border, clearly darker out area
- intro overlay with control symbols; first tap dismisses it and unlocks audio
- nginx now serves everything Cache-Control: no-cache so a reload always
  revalidates and picks up new deploys

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Dockerfile

@@ -1,3 +1,6 @@
 FROM nginx:alpine
+# no-cache => browsers revalidate every request (ETag 304s are cheap), so a new
+# deploy is picked up on plain reload. ponytail: switch to hashed filenames if traffic ever matters.
+RUN printf 'server {\n  listen 80;\n  root /usr/share/nginx/html;\n  etag on;\n  location / { add_header Cache-Control "no-cache, must-revalidate"; }\n}\n' > /etc/nginx/conf.d/default.conf
 COPY index.html game.js /usr/share/nginx/html/
 COPY assets /usr/share/nginx/html/assets