Fall back from invalid preset stores (#1402)

tests/test_preset_store_shape.py

@@ -0,0 +1,12 @@
+import json
+
+from src.preset_manager import PresetManager
+
+
+def test_non_object_preset_store_falls_back_to_defaults(tmp_path):
+    (tmp_path / "presets.json").write_text(json.dumps([]))
+
+    manager = PresetManager(str(tmp_path))
+
+    assert manager.presets == PresetManager.DEFAULT_PRESETS
+    assert manager.get("custom")["enabled"] is False