fix WPF relative resource renaming

[ryanroe]

fix #550

[Copilot]

Pull Request Overview

Fixes WPF relative resource renaming by correcting how file names are extracted and passed to the rename method. The change addresses issue #550 by ensuring that only the filename portion (without directory path) is used for renaming operations.

• Extracts filename portion separately from directory path calculation
• Updates rename calls to use filename instead of full path
• Improves variable naming for clarity

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}