feat: add retry functionality for failed downloads

ui/src/app/services/downloads.service.spec.ts

@@ -117,6 +117,14 @@ describe('DownloadsService', () => {
     req.flush({ presets: ['Preset A'] });
   });
 
+  it('retry() posts the failed download id', () => {
+    service.retry('https://example.com/v').subscribe();
+    const req = httpMock.expectOne('retry');
+    expect(req.request.method).toBe('POST');
+    expect(req.request.body).toEqual({ ids: ['https://example.com/v'] });
+    req.flush({ status: 'ok' });
+  });
+
   it('cancelAdd posts to cancel-add', () => {
     service.cancelAdd().subscribe();
     const req = httpMock.expectOne('cancel-add');

ui/src/app/services/downloads.service.ts

@@ -169,6 +169,12 @@ export class DownloadsService {
     );
   }
 
+  public retry(id: string) {
+    return this.http.post<Status>('retry', { ids: [id] }).pipe(
+      catchError(this.handleHTTPError)
+    );
+  }
+
   public startById(ids: string[]) {
     return this.http.post<Status>('start', {ids: ids}).pipe(
       catchError(this.handleHTTPError)