fix(messaging-ui): stop Start-chat/Create buttons stretching full width

The new-conversation + channel-create forms are flex columns, so the submit
button stretched to full width and read as thin. Give it align-self:flex-start
+ a 38px height so it sizes to its label like a normal button.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/styles.css

@@ -506,6 +506,12 @@
   flex-direction: column;
   gap: 10px;
 }
+/* Submit buttons in the stacked create/compose forms size to content, not full width. */
+.miu-newconv .miu-send,
+.miu-channel-create .miu-send {
+  align-self: flex-start;
+  height: 38px;
+}
 .miu-person-row {
   display: flex;
   align-items: center;