Fix broken demo: pin example data fetch to CineMA commit

The ukb/ example images were removed from the CineMA main branch (UK
Biobank data can't be redistributed publicly), so fetching data from
/main/ returned 404 and broke the CMR Views, MAE, LAX Segmentation, and
Landmark tabs. Fetch from the same commit the cinema package is pinned to
in requirements.txt, which still ships the ukb/ data and matches the API.
Also raise on failed downloads so a bad response isn't cached.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app.py

@@ -38,6 +38,11 @@ from tqdm import tqdm
 cache_dir = Path(__file__).parent
 cache_dir.mkdir(parents=True, exist_ok=True)
 
+# Pin example data to the same CineMA commit as the installed `cinema` package
+# (see requirements.txt). The `main` branch no longer ships the `ukb/` example
+# images, so fetching from this commit keeps the data consistent with the API.
+CINEMA_DATA_REF = "dd9c19cfe5f09c26dbf29373f92ced2f9a0648b7"
+
 
 # set device and dtype
 dtype, device = torch.float32, torch.device("cpu")
@@ -57,8 +62,9 @@ theme = gr.themes.Ocean(
 def load_nifti_from_github(name: str) -> sitk.Image:
     path = cache_dir / name
     if not path.exists():
-        image_url = f"https://raw.githubusercontent.com/mathpluscode/CineMA/main/cinema/examples/data/{name}"
+        image_url = f"https://raw.githubusercontent.com/mathpluscode/CineMA/{CINEMA_DATA_REF}/cinema/examples/data/{name}"
         response = requests.get(image_url)
+        response.raise_for_status()
         path.parent.mkdir(parents=True, exist_ok=True)
         with open(path, "wb") as f:
             f.write(response.content)