Verify Windows registration cleanup after uninstall

[sridharkalaibala]

A GSHCL uninstall removed its application folder but left Windows uninstall and startup entries behind. Add a final cleanup section that uses the explicit per-user registry hive and the build's registry view, then checks removal before allowing the success page. It refuses to unregister a live application or metadata pointing to another installation.

The keep-data choices are unchanged. This is local Windows installation metadata; it does not change cloud data or synchronization.

Validation: 17 setup/recovery/Windows tests passed, including all eight keep combinations, recovery failure protection, and a compiled NSIS fixture covering stale, absent, live, and unrelated registrations with a changed shell context. Five uninstall wording tests and attribution checks passed. Both GSHCL and standard Posnic installers rebuilt successfully and passed package verification. A complete interactive uninstall of the rebuilt package remains to be verified: desktop automation blocked launching the uninstaller. The precise cause of the original interrupted cleanup is not established.

[github-actions]

Merged to develop. Anyone can test this - you do not need write access.

Try it at https://develop.posnic.io, or run it yourself:

git fetch origin develop && git checkout develop
npm install && npm --prefix api install
npm run dev   # then http://localhost:3000

When you have tested it, say what you did and what happened, and set
QA passed or QA failed. If you cannot set labels, just comment -
a maintainer will.

Reporting that something is broken is as useful as fixing it. It is
better found here than by a shopkeeper.