Fix Windows drive-root install directory

[wisdomqin]

What changed

• wait until the actual NSIS directory page exists before binding the path-change handler
• normalize a selected drive root such as D: or D:\ to D:\DSH Desktop
• extend the release contract test to cover the delayed directory-page binding and fixed folder name

Root cause

The previous MUI_PAGE_CUSTOMFUNCTION_SHOW callback was consumed by electron-builder's earlier install-mode page. The directory page therefore never received the handler, so selecting a drive root left the field at D:\ and disabled installation.

Validation

• npm test -- --run — 81 tests passed
• npm run typecheck
• cross-built the Windows x64 NSIS development installer successfully with electron-builder 26.15.3