test: add comprehensive test suite for visualization and comparison modules

tests/conftest.py

@@ -0,0 +1,12 @@
+"""Shared pytest fixtures."""
+import pytest
+from PySide6.QtWidgets import QApplication
+
+
+@pytest.fixture(scope="session")
+def qapp():
+    """Provide a QApplication instance for the test session."""
+    app = QApplication.instance()
+    if app is None:
+        app = QApplication([])
+    yield app