fix: allow Windows drive-root installation

[wisdomqin]

What changed

• add an NSIS directory-page hook for assisted Windows installs
• turn a selected drive root such as D:\ into D:\DSH Desktop
• reuse the configured product filename so development builds use D:\DSH Desktop Dev
• keep explicitly selected non-root directories unchanged
• add release-contract coverage for the custom installer include

Why

NSIS rejects a bare drive root on the directory page before electron-builder's later install-time path sanitization runs. As a result, selecting D:\ disables the Install button even though installing into a child application directory is valid.

The new hook normalizes drive roots as soon as the directory field changes, allowing users to choose a drive without manually creating or typing an application subdirectory.

Validation

• npm test -- --run — 80 tests passed
• npm run typecheck
• npm run build
• formal Windows x64 NSIS installer compiled successfully
• development Windows x64 NSIS installer compiled successfully
• git diff --check