新增任务分享/导入功能并修复只读文件保存 EPERM

- flows: writeFlowFile 覆盖写加固(清只读属性->删除后 rename->覆盖写),
  外部复制导入的只读任务文件此前导致 flows:save EPERM 崩溃
- flows: exportFlowToPath/importFlowFromPath, 分享文件为单个 FlowDefinition
  JSON(.rpaflow.json); 导入校验 schemaVersion/nodes/edges, id 冲突时生成
  imported_ 前缀新 id 绝不覆盖既有任务
- IPC: flows:export 弹保存对话框 / flows:import 弹多选打开对话框,
  失败文件逐个汇报不阻塞其余
- preload: flows.export/flows.import; shared: FlowsImportResult 类型
- TasksPage: 头部「导入任务」按钮 + 每行「分享」按钮; App 接线与 toast
- 测试: 导出/导入往返、id 冲突、非法文件原因、只读覆盖共 4 例(7/7 绿)

src/shared/types.ts

@@ -33,6 +33,12 @@ export interface FlowSaveOptions {
 export type FlowSaveResult =
   | { conflict: false; flow: FlowDefinition }
   | { conflict: true; diskFlow: FlowDefinition };
+
+/** 批量导入任务的结果：成功与失败分开汇报，单文件失败不阻塞其余 */
+export interface FlowsImportResult {
+  imported: FlowSummary[];
+  failed: Array<{ file: string; reason: string }>;
+}
 export interface FlowSummary {
   id: string;
   name: string;