Show which build the start screen is running

"Am I on the right version?" needs an answer on the device, not in a terminal.
The browser cannot see git and push = deploy leaves no build step to inject a
sha into, so the marker is the served main.js's own Last-Modified — it moves on
every deploy with nothing to remember to bump. Match it with
`git log -1 --format=%cd`.

Cache-buster bumped with it: without that the phone keeps the old main.js and
shows a stale build line, which is the exact thing this is meant to catch.

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

index.html

@@ -64,6 +64,8 @@
   #intro .how p { margin: 0; }
   #intro .how span { display: inline-block; width: 30px; }
   #intro em { color: #8a96ab; font-style: normal; font-size: 12px; }
+  /* which build is served — filled in by main.js, empty (and invisible) if the header is missing */
+  #build { margin: 10px 0 0; font-size: 9px; color: #5a6270; letter-spacing: 0.5px; }
   #intro .start { margin-top: 18px; color: #e0cc72; font-weight: bold; letter-spacing: 1px; animation: blink 1.1s step-end infinite; }
   @keyframes blink { 50% { opacity: 0.25; } }
 </style>
@@ -84,8 +86,9 @@
       <p class="kb"><span>⌨️</span> WASD / arrows + SPACE</p>
     </div>
     <p class="start">TAP TO START</p>
+    <p id="build"></p>
   </div>
 </div>
-<script type="module" src="main.js?v=50"></script>
+<script type="module" src="main.js?v=51"></script>
 </body>
 </html>