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/style.css

@@ -340,6 +340,8 @@ body.state-sleeping   #row-speed,
 body.state-sleeping   #row-tokens,
 body.state-sleeping   #row-load-stage { display: none; }
 
+body:not(.state-sleeping) #row-sleep-timer { display: none; }
+
 body.state-waiting    #row-prompt,
 body.state-waiting    #row-speed,
 body.state-waiting    #row-tokens,