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>

log_parser.py

@@ -51,6 +51,7 @@ class ServerState:
     loading: Optional[LoadingInfo] = None
     metrics: Metrics = field(default_factory=Metrics)
     request_count: int = 0
+    sleeping_since: Optional[str] = None  # ISO timestamp when sleeping started
 
     def to_dict(self) -> dict:
         return {
@@ -59,6 +60,7 @@ class ServerState:
             "loading": asdict(self.loading) if self.loading else None,
             "metrics": asdict(self.metrics),
             "request_count": self.request_count,
+            "sleeping_since": self.sleeping_since,
             "timestamp": datetime.now().isoformat(timespec="seconds"),
         }
 
@@ -215,12 +217,17 @@ class LogParser:
                 self._state.model = model_id
             return self._set(state="idle", loading=None)
         if s == "sleeping":
-            return self._set(state="sleeping")
+            return self._set(
+                state="sleeping",
+                sleeping_since=datetime.now().isoformat(timespec="seconds"),
+            )
         return None
 
     # ── Helpers ──────────────────────────────────────────────────────────────
 
     def _set(self, **kwargs) -> ServerState:
+        if "state" in kwargs and kwargs["state"] != "sleeping":
+            self._state.sleeping_since = None
         for k, v in kwargs.items():
             setattr(self._state, k, v)
         return self._state