docs(cleanup): remove 'current' as special provider; align UI/messages and migration naming to 'default' and one-time import rule

- App: update auto-import message to '默认供应商'
- README: clarify default import only when providers are empty
- Plan doc: replace 'current entry' wording with current pointer (manager.current)
- Migration: name live-imported item 'default' instead of 'current'

src/App.tsx

@@ -80,7 +80,7 @@ function App() {
     setProviders(loadedProviders);
     setCurrentProviderId(currentId);
 
-    // 如果供应商列表为空，尝试自动导入现有配置为"current"供应商
+    // 如果供应商列表为空，尝试自动从 live 导入一条默认供应商
     if (Object.keys(loadedProviders).length === 0) {
       await handleAutoImportDefault();
     }
@@ -154,14 +154,14 @@ function App() {
     }
   };
 
-  // 自动导入现有配置为"current"供应商
+  // 自动从 live 导入一条默认供应商（仅首次初始化时）
   const handleAutoImportDefault = async () => {
     try {
       const result = await window.api.importCurrentConfigAsDefault(activeApp);
 
       if (result.success) {
         await loadProviders();
-        showNotification("已自动导入现有配置为 current 供应商", "success", 3000);
+        showNotification("已从现有配置创建默认供应商", "success", 3000);
       }
       // 如果导入失败（比如没有现有配置），静默处理，不显示错误
     } catch (error) {