fix: unify tender countdowns and add monthly calendar navigation

src/hooks/useDeadlineClock.js

@@ -0,0 +1,10 @@
+import {useEffect, useState} from 'react';
+// Keep expiry labels current even when the user leaves a screen open.
+export function useDeadlineClock() {
+  const [now,setNow] = useState(() => new Date());
+  useEffect(() => {
+    const timer = setInterval(() => setNow(new Date()),30000);
+    return () => clearInterval(timer);
+  },[]);
+  return now;
+}