fix(chat): prevent New Chat button from stretching full height

The .btn-new-conv class has flex:1 for the sidebar row layout, but
when reused inside .no-conversation (a column flex), it stretched
vertically to fill the entire viewport — appearing as a giant
purple rectangle. Override with flex:none in the no-conversation
context.

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

frontend/src/views/ChatView.vue

@@ -639,6 +639,9 @@ onUnmounted(() => {
   gap: 1rem;
   color: var(--color-text-muted);
 }
+.no-conversation .btn-new-conv {
+  flex: none;
+}
 
 .empty-msg {
   color: var(--color-text-muted);