fix: support encrypted Windows settings directories

[Rhys-liuliu]

Summary

• handle ERROR_NOT_SAME_DEVICE from EFS-encrypted Windows app-data directories
• preserve the existing atomic replacement path for normal filesystems
• use a flushed backup/copy/rollback fallback and clean up temporary files
• add coverage for fallback replacement and cleanup

Why

On Windows systems where %LOCALAPPDATA%\SplatStudio is EFS-encrypted, MoveFileExW can return ERROR_NOT_SAME_DEVICE even when the temporary and destination files are in the same directory. This leaves project-index.json.tmp, causes get_project_overview to reject, and leaves the UI stuck at "Detecting COLMAP GPU acceleration".

Testing

• npm test (48 passed)
• npm run build
• cargo test --manifest-path src-tauri\Cargo.toml (67 passed)
• cargo clippy --manifest-path src-tauri\Cargo.toml --all-targets -- -D warnings
• npm run verify:engines
• npm run verify:licenses
• Manual: Windows x64 with EFS-encrypted %LOCALAPPDATA%\SplatStudio; verified project-index.json updates, no .tmp/.bak files remain, and COLMAP GPU detection completes on an RTX 5060 Laptop GPU.

Closes #25