Remove border-top on attached segment after attached message

The :not(.message) exclusion caused the segment's gray border-top
to overlap and hide the message's colored border-bottom.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: silverwind

web_src/css/modules/segment.css

@@ -140,7 +140,7 @@
   background: var(--color-box-body);
   color: var(--color-text);
 }
-.ui.attached:not(.message) + .ui.attached.segment:not(.top) {
+.ui.attached + .ui.attached.segment:not(.top) {
   border-top: none;
 }