fix(installer): update electron-builder URLs

[theonlychant]

Summary

Updates stale electron-builder URLs in installer docs and NSIS script
to point to current download locations.

Why

The electron-builder download URLs referenced in the docs and installer
script had changed, causing broken links and potential installer failures
for contributors following the setup guide.

Linked issue

Refs #889

Changes

• Updated electron-builder URL in docs/deployment/code-signing.mdx
• Updated electron-builder URL in docs/plans/desktop-installer.mdx
• Updated electron-builder URL in installer/nsis/installer.nsh

Test plan

• Links manually verified to resolve correctly
• python util/lint.py --all - no failures

Checklist

• I have linked a GitHub issue above (Refs #889).
• I have described why this change is being made, not just what changed.
• I have run linting and tests locally.
• Documentation updated where applicable.