fix(diagnostics): count the 24 hours from the moment the level was raised

The countdown's clock was the moment Settings mounted, so a level raised
two minutes after opening the page read "24 h 2 m" until the first tick
(measured on prod). The clock is read at render; the interval only
re-renders.

frontend/src/components/DiagnosticsSettings.tsx

@@ -15,14 +15,17 @@ const LEVELS: readonly DiagnosticsLevel[] = ["INFO", "DEBUG", "TRACE"];
 
 /** Hours and minutes until `until`, refreshed every 30 s — the status line counts down. */
 function useRemaining(until: string | null): { h: number; m: number } | null {
-  const [now, setNow] = useState(() => Date.now());
+  // The clock is read at render, and the interval only re-renders: a `now` held in state was the
+  // moment the page MOUNTED, so a level raised two minutes after opening Settings read "24 h 2 m"
+  // until the first tick (prod UAT 2026-09-19).
+  const [, tick] = useState(0);
   useEffect(() => {
     if (!until) return;
-    const id = window.setInterval(() => setNow(Date.now()), 30_000);
+    const id = window.setInterval(() => tick((n) => n + 1), 30_000);
     return () => window.clearInterval(id);
   }, [until]);
   if (!until) return null;
-  const minutes = Math.max(0, Math.ceil((Date.parse(until) - now) / 60_000));
+  const minutes = Math.max(0, Math.ceil((Date.parse(until) - Date.now()) / 60_000));
   return { h: Math.floor(minutes / 60), m: minutes % 60 };
 }