fix(mobile): handle file picker cancellation

mobile-uni/src/utils/file-selection.ts

@@ -0,0 +1,28 @@
+export const FILE_SELECTION_CANCELLED = 'FILE_SELECTION_CANCELLED';
+
+export class FileSelectionCancelledError extends Error {
+  readonly code = FILE_SELECTION_CANCELLED;
+
+  constructor(label: string) {
+    super(`未选择${label}`);
+    this.name = 'FileSelectionCancelledError';
+  }
+}
+
+const failureSignal = (failure: unknown): string => {
+  if (typeof failure === 'string') return failure;
+  if (!failure || typeof failure !== 'object') return '';
+  const value = failure as { errMsg?: unknown; message?: unknown };
+  return String(value.errMsg || value.message || '');
+};
+
+export const fileSelectionFailure = (failure: unknown, label: string): Error => {
+  const signal = failureSignal(failure).toLowerCase();
+  if (signal.includes('cancel')) return new FileSelectionCancelledError(label);
+  return new Error(`无法选择${label}，请重试`);
+};
+
+export const isFileSelectionCancelled = (error: unknown): boolean =>
+  error instanceof FileSelectionCancelledError
+  || Boolean(error && typeof error === 'object'
+    && (error as { code?: unknown }).code === FILE_SELECTION_CANCELLED);