fix: clear date selection when clicking an unavailable/closed day

bits-ui blocks onValueChange for unavailable dates, leaving stale selection. Adds native click detection on [data-unavailable] elements to call onchange(undefined) so the time picker hides and the user knows nothing was picked.

frontend/src/lib/components/account/EditRequestModal.svelte

@@ -485,6 +485,21 @@
 		}
 	});
 
+	// Clear selection when navigating to a month that doesn't contain the selected date.
+	// Runs AFTER all synchronous state changes settle, so clicking a date in a different
+	// month (fires both onPlaceholderChange and onValueChange) keeps the new selection,
+	// while clicking prev/next arrows without picking a date clears it.
+	$effect(() => {
+		if (
+			newDate &&
+			placeholderDate &&
+			(newDate.month !== placeholderDate.month || newDate.year !== placeholderDate.year)
+		) {
+			newDate = undefined;
+			newTime = '';
+		}
+	});
+
 	// ─── API calls ──────────────────────────────────────────
 	async function fetchHoursRange(startDate: CalendarDate, months: number) {
 		loadingHours = true;