Fix custom Windows install directory display

[wisdomqin]

What changed

• locate the visible NSIS directory page instead of binding to electron-builder's hidden install-mode page
• show the final application directory for every custom parent selection, for example D:\DSH Desktop in a formal build and D:\DSH Desktop Dev in a development preview
• support drive roots, arbitrary subdirectories, Browse selections, and manually entered paths after the field loses focus
• keep the directory-page watcher valid when navigating backward and forward in the installer
• preserve electron-builder's existing install-time path safeguard

Root cause

The earlier fix only recognized drive roots and its page watcher stopped at the first child dialog. Electron-builder retains previous custom pages as hidden dialogs, so the handler could bind to the wrong page and never update the visible directory field.

Validation

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

The final visible-field behavior still needs confirmation with this PR's preview installer on Windows.

[wisdomqin]

Windows validation preview: desktop-preview-20260818.1

Direct installer: dsh-desktop-dev-windows-x64-setup.exe

Built from commit 1a81095136c8e248720aa1cbdd148bdfe39c06f6. The Windows job passed tests, typecheck, both package builds, and the packaged Harness smoke test. Manual Windows confirmation is still required for the visible custom-directory behavior.

[wisdomqin]

Closing this draft because the custom-directory display behavior still did not work reliably in Windows real-machine testing. This experiment was never merged into main; the existing drive-root compatibility already present on main is unaffected. We can revisit the installer UX later if it becomes important.