msglist: Fix recipient header date alignment for RTL layouts.

Previously, the TODAY (and other date) labels in recipient headers used, which applied
asymmetric left/right padding only in LTR layouts. This caused the label to be misaligned
in RTL layouts, as seen in screenshots from #2005.

Change to use , so the start/end padding swaps correctly in RTL, matching the alignment
of message timestamps and improving visual consistency.

Author: yash-agarwa-l

lib/widgets/message_list.dart

@@ -1953,7 +1953,7 @@ class RecipientHeaderDate extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return Padding(
-      padding: const EdgeInsets.fromLTRB(10, 0, 16, 0),
+      padding: const EdgeInsetsDirectional.fromSTEB(10, 0, 16, 0),
       child: DateText(
         fontSize: 16,
         // In Figma this has a line-height of 19, but using 18