test: isolate QSettings so user preferences cannot fail GUI tests

MainWindow persists preferences (window layout, canvas backend) through
QSettings("OTKO", "OTKO"), so tests constructing it were reading the
developer's real settings: after switching to the Plotly backend the
PyVista-specific viewport-axis tests failed with "PlotlyCanvas has no
attribute renderer". A session fixture now redirects QSettings into a
temp dir and is skipped when Qt is not installed, keeping the headless
job Qt-free.

tests/conftest.py

@@ -11,6 +11,7 @@ domain after every test. The import is inside the teardown so that
 
 from __future__ import annotations
 
+import importlib.util
 import os
 from collections.abc import Iterator
 
@@ -19,6 +20,27 @@ import pytest
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
 
 
+@pytest.fixture(autouse=True, scope="session")
+def _isolate_qsettings(tmp_path_factory: pytest.TempPathFactory) -> Iterator[None]:
+    """Redirect ``QSettings`` to a temp dir so tests never touch user state.
+
+    ``MainWindow`` persists preferences (window layout, canvas backend) under
+    ``QSettings("OTKO", "OTKO")``. Without redirection a developer who, say,
+    switched to the Plotly backend would see PyVista-specific GUI tests fail
+    against their real settings. Skipped entirely when Qt is not installed, so
+    the headless job stays free of Qt.
+    """
+    if importlib.util.find_spec("PySide6") is None:
+        yield
+        return
+    from PySide6.QtCore import QSettings
+
+    settings_dir = tmp_path_factory.mktemp("qsettings")
+    for fmt in (QSettings.Format.NativeFormat, QSettings.Format.IniFormat):
+        QSettings.setPath(fmt, QSettings.Scope.UserScope, str(settings_dir))
+    yield
+
+
 @pytest.fixture(autouse=True)
 def _wipe_opensees_domain() -> Iterator[None]:
     """Wipe the OpenseesPy domain after a test if the solver was imported."""