Better License Management

A couple of customers have emailed me to request better license management. Especially for team licenses, it would be convenient to see how many licenses are currently in use. It would probably also be convenient to move licenses between computers.

I'm not sure how to best implement this. At the moment, Postico only contacts a server to report license activation, but there is no way to "deactivate" a license, and there's also no way to see which licenses are still in use.

Basic features that big customers need:

- View Number of activated licenses
- Deactivate licenses (eg. when you get a new computer)
- See when a license was last used (eg. within the last month) to identify unused licenses

I'm also not sure how much info the license manager should show? For privacy reasons, it should probably not show every single time that Postico checks the license, but some coarse-grained info (eg. "used in the last month") would probably be really useful.

No files or tests are named. Start by reviewing the existing server-side license activation flow described in the issue, then define the license-management scope and privacy behavior; done should cover activation counts, deactivation, and coarse last-use information.

Written by the indexing model from the issue text.