Update knowledge base options in AssistantPage for improved clarity

Changed the default knowledge base value from "政务政策知识库" to "无" and updated the options list to include "无" as the first choice. This adjustment enhances user understanding and provides a clearer selection for knowledge base configurations.

src/components/pages/AssistantPage.tsx

@@ -206,7 +206,7 @@ export function AssistantPage() {
     model: "DeepSeek-V3",
     asr: "SenseVoice",
     voice: "晓宁",
-    knowledgeBase: "政务政策知识库",
+    knowledgeBase: "无",
     enableInterrupt: true,
   });
   const [view, setView] = useState<"list" | "choose" | "create" | "placeholder">(
@@ -862,7 +862,7 @@ export function AssistantPage() {
               label="知识库"
               value={form.knowledgeBase}
               onChange={(value) => updateForm("knowledgeBase", value)}
-              options={["政务政策知识库", "售后知识库", "教育课程知识库", "医疗问答知识库"]}
+              options={["无", "政务政策知识库", "售后知识库", "教育课程知识库", "医疗问答知识库"]}
             />
           </SectionCard>