fix: add null check for chatStore.conversations in StickyModeHeader

Optional chaining prevents TypeError when test mocks don't provide conversations array.

src/agentkit/server/frontend/src/components/chat/StickyModeHeader.vue

@@ -159,7 +159,7 @@ const allExperts = computed<IExpertDisplay[]>(() => {
     // 中保存的 YAML color。
     const liveColorByName = new Map<string, string>()
     const liveAvatarByName = new Map<string, string>()
-    const conv = chatStore.conversations.find(
+    const conv = chatStore.conversations?.find(
       (c: { id: string; messages?: unknown[] }) => c.id === chatStore.currentConversationId,
     )
     if (conv?.messages) {