fix(chat): let backend auto-generate conversation titles

KnowledgeView and WorkspaceView were passing explicit titles
("Knowledge chat", "Project — Workspace") to createConversation(),
which prevented the backend's auto-title generation from firing
(condition: `not conv_title`). Pass no title so the background
title generator names conversations after their first message.

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

frontend/src/views/KnowledgeView.vue

@@ -247,7 +247,7 @@ const chatCollapsed = ref(false);
 const chatConvId = ref<number | null>(null);
 async function onMinichatSubmit(payload: { content: string; contextNoteId?: number }) {
   if (!chatConvId.value) {
-    const conv = await chatStore.createConversation("Knowledge chat");
+    const conv = await chatStore.createConversation();
     chatConvId.value = conv.id;
     await chatStore.fetchConversation(conv.id);
   } else if (chatStore.currentConversation?.id !== chatConvId.value) {