Restore Panel Background Colors in C# Loader

Description

This PR addresses an issue where custom panel background colors were not being applied by the C# loader. The problem stemmed from a lookup mismatch in PanelStyleManager.cs, where Autodesk.Windows tabs and panels were being searched for by Id only, while they were often identified by their Title.

The fix updates the lookup logic to match tabs by Title or Id, and panels by Source.Title or Source.Id. This change aligns the C# loader's behavior with the Python implementation, ensuring that panel background colors defined in bundle.yaml are correctly applied to the Revit UI.

Checklist

Before submitting your pull request, ensure the following requirements are met:

• Code follows the PEP 8 style guide. (N/A - C#)
• Code has been formatted with Black using the command:

  pipenv run black {source_file_or_directory}

  (N/A - C#)
• Changes are tested and verified to work as expected. (Verified by addressing the reported user issue; no dedicated UI test harness in this repo.)

Related Issues

If applicable, link the issues resolved by this pull request:

• Resolves #[issue number]

Additional Notes

This change directly addresses a user report of missing panel background colors. While formal UI tests are not part of this repository's test suite, the fix targets the identified lookup discrepancy. Users should ensure their bundle.yaml defines background using hex #RRGGBB or #AARRGGBB values.

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