feat: show a relative hint for the next update in the footer

Recompute the next 06:00 UTC run client-side so the footer stays
correct on a cached page, and append a relative ("in 3 hours") hint.
The build-time absolute date remains as the no-JS fallback.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: kt3k

src/_includes/layouts/base.vto

@@ -37,8 +37,28 @@
         RSS on every repo page
       </span>
       <span aria-hidden="true">·</span>
-      <span>Next update {{ nextUpdate }}</span>
+      <span>Next update <span data-next-update>{{ nextUpdate }}</span></span>
     </div>
   </footer>
+
+  <script>
+  // Recompute the next 06:00 UTC run on the client so the footer stays correct
+  // even on a cached page, and append a relative ("in 3 hours") hint.
+  (function () {
+    var el = document.querySelector("[data-next-update]");
+    if (!el) return;
+    var MONTHS = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
+    var now = new Date();
+    var next = new Date(Date.UTC(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate(), 6, 0, 0));
+    if (next.getTime() <= now.getTime()) next.setUTCDate(next.getUTCDate() + 1);
+    var abs = MONTHS[next.getUTCMonth()] + " " + next.getUTCDate() + ", 06:00 UTC";
+    var mins = Math.round((next.getTime() - now.getTime()) / 60000);
+    var rel;
+    if (mins < 1) rel = "any moment";
+    else if (mins < 60) rel = "in " + mins + " min";
+    else { var h = Math.round(mins / 60); rel = "in " + h + " hour" + (h === 1 ? "" : "s"); }
+    el.textContent = abs + " (" + rel + ")";
+  })();
+  </script>
 </body>
 </html>