Add platform-specific paths for Windows and macOS

[DOVIS4415]

Motivation

Some users work with the same Blender/GoB configuration on both Windows and macOS. The current single ZBrush, GoZ project, and Pixologic public path fields cannot represent both operating systems at once: switching systems leaves GoB with paths that only exist on the other OS.

This also needs to be backward-compatible. Existing users should not have to re-enter their current GoB paths after updating, so legacy single-path preferences are detected by path style and migrated to the matching platform-specific field.

Changes

• Store separate Windows and macOS values for the ZBrush executable, GoZ project directory, and optional Pixologic public directory.
• Use only the current platform's values at runtime while keeping both systems' values available in preferences.
• Keep the legacy preference properties and migrate matching saved values into the new platform-specific properties.
• Group Windows and macOS settings into foldout sections; Windows is shown first and expanded by default.
• Fall back to the platform's normal ZBrush discovery when a configured executable no longer exists.
• Correct macOS discovery to search /Applications and accept a direct ZBrush.app result.

Validation

• Blender 5.2.0 LTS with an isolated official-extension installation.
• Platform selection and legacy migration assertions, including Windows drive-letter paths, UNC paths, macOS paths, project/public path defaults, invalid-path fallback, and foldout UI behavior.
• Extension register/unregister/register smoke test.
• Blender extension source audit, official package build, and package validation.

Windows path behavior was exercised through platform-selection tests; the isolated Blender runtime validation was performed on macOS.