Refresh extension builds (split Chrome/Firefox) ...

Repackage the Chrome and Firefox builds from the extension repo and
update extensions/manifest.json checksums, sizes, and release date.

Co-Authored-By: Claude <noreply@anthropic.com>

Add in-app extension download page ...

Distribute the gnexus-creds browser extension from the service itself.
Package Chrome and Firefox builds into extensions/dist/ with a manifest
describing version, checksums, and sizes; serve them via authenticated
FastAPI endpoints and surface a new Extension tab in the Vue UI with
download cards, SHA-256, and install instructions.

Co-Authored-By: Claude <noreply@anthropic.com>