All-users update: NSIS installer window shows red error icon instead of app icon (cosmetic)

## Summary

During an **all-users** (elevated / per-machine) update, the small window icon (favicon in the title bar) on the NSIS installer window shows a generic **red error icon** instead of the Comfy Desktop app icon. The per-user update flow shows the correct icon.

This is purely **cosmetic** — both per-user and all-users updates install and complete successfully. Filing as low priority per discussion.

## Steps to reproduce

1. Install Comfy Desktop for **all users** (elevated / per-machine).
2. Trigger an update (startup install path).
3. Observe the NSIS installer window during the elevated install — the small title-bar icon is a red error icon.

## Expected

The NSIS window should show the Comfy Desktop app icon, matching the per-user installer.

## Notes / investigation so far

- Not caused by the update-on-startup logic changes (#1066 / #1079 / #1088 / #1093). The icon is correct for per-user installs and only wrong on the elevated/all-users instance.
- Build logs show installer icons are configured correctly (`MUI_ICON=icon.ico`, `MUI_UNICON=icon.ico`).
- Most likely an NSIS/UAC elevation cosmetic quirk: the elevated child instance / temp relaunch loses the small window icon.
- For this app (`perMachine: false`), the all-users path is a rare `/ALLUSERS` provisioning case, so impact is small.

## Possible fixes (future)

1. Explicitly set `installerIcon` in `electron-builder.yml` and ensure the `.ico` includes small sizes (16×16 / 32×32) — cheap first attempt, uncertain.
2. Custom NSIS `WM_SETICON` on `$HWNDPARENT` in the elevated instance — more likely to work but fiddly; requires a real ToDesktop build to verify.

Either requires a real ToDesktop Windows build to validate.

## Priority

Low — cosmetic only, all-users path is rare, updates work correctly.