IOS app should support self-hosted relay connection. Eg. Tailscale

**Motivation**
What problem does this solve? Who runs into it and when?

**Proposed solution**
Describe what you'd like to see.

**Alternatives considered**
What other approaches did you consider?

**Additional context**
Anything else that helps — links, screenshots, prior art.

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Before opening: please [search open issues and PRs](https://github.com/block/buzz/issues?q=is%3Aopen) for duplicates — link the closest one, or say "none found".

The issue names the iOS app, self-hosted relay connections, and Tailscale but provides no files, tests, or entry points. Define the expected connection flow and acceptance criteria first, then locate the relevant mobile networking code and tests.

Written by the indexing model from the issue text.