接入原生壳分享卡图片导出

新增 file.exportImage 宿主能力契约

分享卡下载在原生壳中优先走宿主图片导出

Expo 壳写入缓存图片并调用系统分享保存

Tauri 壳通过保存对话框写入图片字节

补齐能力漂移检查、测试和架构文档

packages/shared/src/contracts/hostBridge.ts

@@ -24,6 +24,7 @@ export const HOST_BRIDGE_METHODS = [
   'app.setTitle',
   'clipboard.writeText',
   'file.exportText',
+  'file.exportImage',
   'haptics.impact',
 ] as const;
 
@@ -171,6 +172,18 @@ export type FileExportTextResult = {
   bytes: number;
 };
 
+export type FileExportImagePayload = {
+  fileName: string;
+  base64Data: string;
+  mimeType: 'image/png' | 'image/jpeg' | 'image/webp';
+};
+
+export type FileExportImageResult = {
+  action: 'saved';
+  fileName: string;
+  bytes: number;
+};
+
 export type HapticsImpactPayload = {
   style?: 'light' | 'medium' | 'heavy';
 };