fix(export): surface lenient per-object failures in export completion

Closes #8184

apps/desktop/src/lib/backend/tauri.ts

@@ -4825,6 +4825,10 @@ export interface ExportProgress {
   error: string | null;
   /** True while listing schema / prefetching metadata before objects are written. */
   preparing?: boolean;
+  /** Per-object failures written into the file as `-- ERROR` comments (lenient mode). */
+  errorCount?: number;
+  /** First lenient failure, for completion warnings without opening the file. */
+  errorSummary?: string | null;
 }
 
 // --- Table Export ---