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/components/ChatPanel.vue

@@ -665,7 +665,6 @@ defineExpose({ focus, prefill, send, contextCount, sidebarOpen, toggleContextSid
 .input-wrapper {
   grid-column: 2;
   grid-row: 2;
-  border-top: 1px solid var(--color-border);
   padding: 0.5rem 1rem 0.75rem;
   display: flex;
   flex-direction: column;