Improve folder generator editor tooling

[emirbesir]

Summary

• refactor folder creation to use Application.dataPath, track skipped folders, and return detailed generation results
• polish the editor window with cached previews, whitespace filtering, and last-run summaries
• move scripts under the UnityFolderGenerator.Editor namespace and add an editor-only assembly definition

Testing

• Not run (Unity Editor tooling)

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https://chatgpt.com/codex/tasks/task_e_68d820a84fcc8320a0d688d029a8d8e0