Task #353: Fix current-meeting highlight sticking after device sleep

The green highlight on the currently-in-progress meeting row would get
stuck when the user locked their iPad, switched apps, or the device
slept. The 60-second setInterval timer that drives `nowTick` is frozen
by the browser during background/sleep states. When the user returned,
the timer wouldn't fire immediately, leaving the highlight on the
old meeting.

Fix: Added a `visibilitychange` event listener alongside the existing
interval timer. When the document becomes visible again, it immediately
calls `setNowTick(Date.now())`, which triggers `computeCurrentMeetingId`
to re-evaluate via the existing useMemo dependency. The listener is
properly cleaned up in the effect's return function.

Changed file:
- artifacts/tx-os/src/pages/executive-meetings.tsx (lines 1065-1084)

artifacts/tx-os/src/pages/executive-meetings.tsx

@@ -1067,9 +1067,16 @@ function ScheduleSection({
       setNowTick(Date.now());
       intervalId = setInterval(() => setNowTick(Date.now()), 60_000);
     }, msToNextMinute);
+    const onVisibilityChange = () => {
+      if (document.visibilityState === "visible") {
+        setNowTick(Date.now());
+      }
+    };
+    document.addEventListener("visibilitychange", onVisibilityChange);
     return () => {
       clearTimeout(timeoutId);
       if (intervalId) clearInterval(intervalId);
+      document.removeEventListener("visibilitychange", onVisibilityChange);
     };
   }, []);