fix(chat): remove input bar border across all chat views

Remove border-top from .input-wrapper in ChatPanel directly instead
of overriding per-view. Applies to chat, workspace, and briefing.

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

frontend/src/views/BriefingView.vue

@@ -442,10 +442,6 @@ onMounted(async () => {
   min-height: 0;
 }
 
-.briefing-chat-panel :deep(.input-wrapper) {
-  border-top: none;
-}
-
 /* ─── Right column (Weather + News) ──────────────────────────────────────── */
 
 .briefing-right {