fix(ui): load NiiVue directly in js_on_load, remove head= dependency (#24)

Root cause: The `head=` parameter injects a <script type="module"> that loads
NiiVue at page load time. If this ES module import fails (404, wrong MIME type,
or network error), it can block Gradio's Svelte app from hydrating, causing the
"Loading..." spinner to show forever.

Fix: Make js_on_load completely self-sufficient for NiiVue loading:
1. Embed NiiVue library URL in HTML via data-niivue-url attribute
2. js_on_load loads NiiVue directly via dynamic import() from this URL
3. Cache loaded Niivue in window.Niivue for subsequent uses
4. Remove head= parameter from launch() - no longer needed

This ensures Gradio always loads (even if NiiVue fails to load), and NiiVue
loading errors are isolated to the viewer component, not blocking the whole app.

Also: Add pyarrow to mypy overrides (proper fix for missing stubs).

All 136 tests pass. Lint, format, mypy clean.

pyproject.toml

@@ -111,6 +111,7 @@ module = [
     "datasets.*",
     "niivue.*",
     "numpy.*",
+    "pyarrow.*",
     "pytest.*",
     # DeepISLES modules (only available in DeepISLES Docker image)
     "src.isles22_ensemble",

src/stroke_deepisles_demo/data/adapter.py

@@ -269,7 +269,7 @@ class HuggingFaceDataset:
         Returns:
             Dict with dwi_bytes, adc_bytes, and optionally mask_bytes
         """
-        import pyarrow.parquet as pq  # type: ignore[import-untyped]
+        import pyarrow.parquet as pq
         from huggingface_hub import HfFileSystem
 
         from stroke_deepisles_demo.data.constants import ISLES24_NUM_FILES