Refactor runner and installation scripts for improved functionality

- Removed the `--disable-hiprt` flag from the runner command, simplifying the rendering options for users.
- Updated the `jiggablend-runner` script and README to reflect the removal of the HIPRT control flag, enhancing clarity in usage instructions.
- Enhanced the installation script to provide clearer examples for running the jiggablend manager and runner, improving user experience during setup.
- Implemented a more robust GPU backend detection mechanism, allowing for better compatibility with various hardware configurations.

internal/runner/api/manager.go

@@ -241,8 +241,8 @@ func (m *ManagerClient) DownloadContext(contextPath, jobToken string) (io.ReadCl
 	}
 
 	if resp.StatusCode != http.StatusOK {
+		defer resp.Body.Close()
 		body, _ := io.ReadAll(resp.Body)
-		resp.Body.Close()
 		return nil, fmt.Errorf("context download failed with status %d: %s", resp.StatusCode, string(body))
 	}
 
@@ -435,8 +435,8 @@ func (m *ManagerClient) DownloadBlender(version string) (io.ReadCloser, error) {
 	}
 
 	if resp.StatusCode != http.StatusOK {
+		defer resp.Body.Close()
 		body, _ := io.ReadAll(resp.Body)
-		resp.Body.Close()
 		return nil, fmt.Errorf("failed to download blender: status %d, body: %s", resp.StatusCode, string(body))
 	}