fix: auto-scroll workspace chat when messages are added

Watch currentConversation.messages.length so the chat panel scrolls to
the bottom when user messages are appended or assistant messages land,
not only while streaming content is updating.

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

frontend/src/views/WorkspaceView.vue

@@ -115,6 +115,7 @@ function scrollToBottom() {
 }
 
 watch(() => chatStore.streamingContent, scrollToBottom);
+watch(() => chatStore.currentConversation?.messages.length, scrollToBottom);
 
 function togglePanel(panel: keyof typeof panelOpen.value) {
   const open = panelOpen.value;