Add default instance auto-connect on startup

[Faerbit]

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📖 Description

Add default instance auto-connect on startup

Adds a settings dropdown to configure which instance should be automatically connected when the client launches. A companion checkbox enables triggering MFA prompts during auto-connect for locations that require it (disabled by default). Selecting "None" (the default) preserves the existing behaviour of no auto-connect.

Relates to #606. Doesn't necessarily close it, as it only connects to one instance.

🛠️ Dev Branch Merge Checklist:

Documentation

• If testing requires changes in the environment or deployment, please update the documentation (https://docs.defguard.net) first and attach the link to the documentation section in this pull request
• I have commented on my code, particularly in hard-to-understand areas

Docs: #62

Testing

• I have performed manual tests manually and all changes work
• New and existing unit tests pass locally with my changes

I was able to successfully test my changes on the main branch. I rebased the changes to the dev branch, because I assume this is the preferred development workflow. I'm unable to verify my changes on the dev branch, because it currently doesn't build.

🏚️ Main Branch Merge Checklist:

Testing

• I have merged my changes before to dev and the dev checklist is done
• I have tested all functionalities on the dev instance and they work

Documentation

• I have made corresponding changes to the user & admin documentation and added new features documentation with screenshots for users/admins

[Faerbit]

I would like to agree to your CA, but cannot read it as the link 404s.

[teon]

Thank you @Faerbit for your contribution 🫡

We are in the middle of 2.0 release and have much workflow - as we would like to release it this week.

Well get back to review this PR after the release!