fix(frontend): make the jalali expiry clear button actually clear

persian-calendar-suite seeds today's date and emits it whenever it mounts
without a value. Clearing the expiry remounts the picker with a null value,
so the library immediately fired onChange(today) and the date came straight
back — and it also painted that seeded date into its read-only input.

Swallow the mount-time emit (re-armed on every clear-remount) and hide the
seeded text while the value is empty, so a cleared expiry stays empty and a
fresh client/inbound form no longer silently adopts today as its expiry.

frontend/src/components/form/DateTimePicker.css

@@ -69,3 +69,9 @@
 .jdp-ultra .jdp-clear:hover {
   color: rgba(255, 255, 255, 0.45);
 }
+
+/* With no value the library still paints today's date into its readOnly input;
+   hide it so an empty (or just-cleared) expiry actually looks empty. */
+.jdp-wrap.jdp-empty input {
+  color: transparent !important;
+}