feat: sleep duration timer in stats panel

Track sleeping_since (ISO timestamp) in ServerState; auto-cleared by _set()
whenever state leaves sleeping. Frontend runs a 1s interval showing elapsed
time as "Xs", "Xm Ys", or "Xh Ym" in a new ASLEEP stat row (visible only
during state-sleeping). Timer survives page reload via server-side timestamp.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/index.html

@@ -147,6 +147,10 @@
         <span class="sk">STAGE</span>
         <span class="sv" id="sv-load-stage">—</span>
       </div>
+      <div class="stat-row" id="row-sleep-timer">
+        <span class="sk">ASLEEP</span>
+        <span class="sv" id="sv-sleep-timer">—</span>
+      </div>
       <div class="stat-row" id="row-reqs">
         <span class="sk">REQS</span>
         <span class="sv" id="sv-reqs">0</span>