fix: AdminView 确保 search config 正确保存

- 添加默认值处理
- vlm_min_score: 默认 0.0
- vlm_max_results: 默认 5
- vlm_output_language: 默认 'en'
- 修复空字符串导致保存失败的问题

src/views/AdminView.vue

@@ -1148,7 +1148,11 @@ async function saveConfig() {
       processor_config: processorConfig.value,
       filter_config: filterConfig.value,
       system_config: systemConfig.value,
-      search: searchConfig.value,
+      search: {
+        vlm_min_score: searchConfig.value.vlm_min_score || 0.0,
+        vlm_max_results: searchConfig.value.vlm_max_results || 5,
+        vlm_output_language: searchConfig.value.vlm_output_language || 'en'
+      },
     }
     const res = await fetch('/api/v1/admin/config', {
       method: 'POST',