msglist: Adjust layout and spacing in messages

Update per Figma design. Messages have 4px vertical padding
and 16px horizontal padding, and if the sender is present
it adds 2px to the top padding and 4px spacing between the
sender and the message contents.

Author: sirpengi

lib/widgets/message_list.dart

@@ -895,23 +895,23 @@ class MessageWithPossibleSender extends StatelessWidget {
       behavior: HitTestBehavior.translucent,
       onLongPress: () => showMessageActionSheet(context: context, message: message),
       child: Padding(
-        padding: const EdgeInsets.only(top: 2, bottom: 3, left: 8, right: 8),
+        padding: const EdgeInsets.symmetric(vertical: 4),
         child: Row(crossAxisAlignment: CrossAxisAlignment.start, children: [
-          const SizedBox(width: 3 + 35 + 11),
+          const SizedBox(width: 16),
           Expanded(
             child: Column(
               crossAxisAlignment: CrossAxisAlignment.stretch,
               children: [
-                if (item.showSender) ...[
-                  const SizedBox(height: 3),
-                  if (senderRow != null) senderRow,
+                if (senderRow != null) ...[
+                  const SizedBox(height: 2),
+                  senderRow,
                   const SizedBox(height: 4),
                 ],
                 MessageContent(message: message, content: item.content),
                 if ((message.reactions?.total ?? 0) > 0)
                   ReactionChipsList(messageId: message.id, reactions: message.reactions!)
               ])),
-          const SizedBox(width: 80),
+          const SizedBox(width: 16),
         ])));
   }
 }