Auto-trigger locked-files restoration exactly at the 60s deadline

Previously the restore check only ran lazily on a player's next
message, so files could stay wrongly-removed for an arbitrary amount
of time after the window expired. The frontend now schedules a
setTimeout (using the server-provided deadline, mirroring the terminal
lock's reveal timer) that pings the backend right at the deadline so
the check runs promptly regardless of player activity. Restored via
data-files-removal-deadline on page load too, so a refresh mid-window
doesn't lose the timer.

templates/game/index.html.twig

@@ -26,6 +26,9 @@
              data-lock-reveal-at="{{ lock.revealAt }}"
              data-lock-unlock-at="{{ lock.unlockAt }}"
              {% endif %}
+             {% if filesRemovalDeadline %}
+             data-files-removal-deadline="{{ filesRemovalDeadline }}"
+             {% endif %}
              style="display:none">
         </div>