Complete multi-scene management and publish Beta 6

[WebCrew]

Summary

• complete the Multi-Scene Project Browser with create, rename and move workflows
• support validated scene subfolders inside scenes/
• reject path escapes, symbolic-link traversal and silent overwrites
• update the startup scene reference when its્ઁ file is moved
• retain unsaved-change protection during scene operations
• version the application and installer as 0.5.0-beta.6
• add a repeatable Beta 6 Windows acceptance checklist

Verification

• node scripts/check.cjs
• syntax checks for main, project paths, preload and renderer
• git diff --check

The Windows workflow performs the packaged Electron smoke test and installer build.