fix: period tracker day count off by one due to timezone mismatch

ongoingDay compared today (local time with hours) against startDate
parsed as UTC midnight, causing Math.floor to undershoot by 1 day.
Use todayMidnight and parseLocal to normalize both to local midnight.

src/lib/components/fitness/PeriodTracker.svelte

@@ -246,8 +246,8 @@
 	// Days into current period
 	const ongoingDay = $derived.by(() => {
 		if (!ongoing) return 0;
-		const start = new Date(ongoing.startDate);
-		return Math.floor((today.getTime() - start.getTime()) / 86400000) + 1;
+		const start = parseLocal(ongoing.startDate);
+		return Math.floor((todayMidnight - start) / 86400000) + 1;
 	});
 
 	// Calendar data