Fix magic link login + Don't elevate + Improve extension loading reliability

A bunch of changes.
The installer now prompts you to uninstall the original client, so that magic link login works again.
It also no longer runs elevated unless it's required to update, in which case it only elevates temporarily and never launches the client itself elevated (fixes a bunch of issues).
Additionally, I've added a "sentinel" extension which will trigger the page to reload, to fix some instances where extensions are being stubborn about not loading.

Overall, this should make the whole thing more reliable.