Add skipif marker for Excel-dependent colorbar test

- Add conditional test marker to skip colorbar tests when Excel is not available
- Import is_app_available utility from xlviews.testing
- Ensure test suite can handle environments without Excel installation

Author: daizutabi

tests/dataframes/test_colorbar.py

@@ -2,6 +2,9 @@
 from xlwings import Sheet
 
 from xlviews.core.range import Range
+from xlviews.testing import is_app_available
+
+pytestmark = pytest.mark.skipif(not is_app_available(), reason="Excel not installed")
 
 
 @pytest.fixture(scope="module")