Import/sync user settings from another Weblate instance/server/deployment

### Describe the problem

It is common for people who translate one project to want to translate another, which may be self-hosting a Weblate instance.
It is annoying to create and set an account on each of them (selecting languages to translate and secondary languages, workflow preferences, external links).

### Describe the solution you'd like

A way to sync setting between different instances. Maybe a file export-import, or a direct cross-origin HTTP request.

### Describe alternatives you've considered

_No response_

### Screenshots

_No response_

### Additional context

Need to make sure that it's safe to import settings from a malicious source.

No implementation files, tests, or entry points are named. Start by locating the user settings model and any existing settings import or cross-origin interfaces, then review how language, workflow, and external-link preferences are stored. Done requires an agreed synchronization design with explicit protection against importing settings from a malicious source.

Written by the indexing model from the issue text.