fix redundant dates showing on Label tab

using isoDatesDifference here (which wraps Luxon's diff) yields decimal values when a full datetime string is provided. If on the same day we are expecting the diff to be 0 (falsy) but get 0.2 or similar (truthy)
All we actually need here is to know if the start and end fall on different dates, so we can simply take the YYYY-MM-DD part of the strings and test for string equality
KISS

www/js/diary/useDerivedProperties.tsx

@@ -18,7 +18,8 @@ const useDerivedProperties = (tlEntry) => {
     const endFmt = tlEntry.end_fmt_time || tlEntry.exit_fmt_time;
     const beginDt = tlEntry.start_local_dt || tlEntry.enter_local_dt;
     const endDt = tlEntry.end_local_dt || tlEntry.exit_local_dt;
-    const tlEntryIsMultiDay = isoDatesDifference(beginFmt, endFmt);
+    // given YYYY-MM-DDTHH:MM:SSZ strings: if YYYY-MM-DD differs, is multi-day
+    const tlEntryIsMultiDay = beginFmt.substring(0, 10) != endFmt.substring(0, 10);
 
     return {
       confirmedMode: confirmedModeFor(tlEntry),