fix(ui): use lazy imports to avoid RuntimeWarning on startup (#13)

Problem:
When running `python -m stroke_deepisles_demo.ui.app`, Python imports
the ui package first, which triggered `ui/__init__.py` to import from
`ui.app`. This caused a RuntimeWarning:
"'stroke_deepisles_demo.ui.app' found in sys.modules after import
of package 'stroke_deepisles_demo.ui', but prior to execution"

Solution:
Use `__getattr__` for lazy imports in `ui/__init__.py`. This defers
the import of `create_app` and `get_demo` until they're actually
accessed, avoiding the circular import issue.

This is a Python best practice for packages with runnable modules.

src/stroke_deepisles_demo/ui/__init__.py

@@ -1,5 +1,20 @@
-"""UI module for stroke-deepisles-demo."""
+"""UI module for stroke-deepisles-demo.
+
+Exports:
+    create_app: Create the Gradio application
+    get_demo: Get the global demo instance (lazy initialization)
+"""
+
+from typing import Any
+
+
+def __getattr__(name: str) -> Any:
+    """Lazy import to avoid circular import when running as `python -m`."""
+    if name in ("create_app", "get_demo"):
+        from stroke_deepisles_demo.ui.app import create_app, get_demo
+
+        return {"create_app": create_app, "get_demo": get_demo}[name]
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
 
-from stroke_deepisles_demo.ui.app import create_app, get_demo
 
 __all__ = ["create_app", "get_demo"]