Indicate relayed tunnels and fix a build problem

[nbrownus]

Adds some basic relay information for tunnels using a relay

Also ran into a build issue from our ios target getting moved back to 13, this asserts we are at the correct version (14)

[jasikpark]

Reviewed with most of the attention on the Dart↔Go contract, since none of the Go in this repo changes.

Verified:

• The JSON keys match nebula v1.11.0's ControlHostInfo tags exactly (currentRelaysToMe/currentRelaysThroughMe, []netip.Addr → string arrays), so the already-pinned version emits everything the model parses — no Go-side changes needed, as expected.
• isRelayed semantics check out against copyHostInfo: relay IPs persist in relay state after a direct path exists, and a relayed tunnel's CurrentRemote is the zero AddrPort, which marshals to "" → null on the Dart side. Gating on currentRemote == null && currentRelaysToMe.isNotEmpty is exactly right.

praise: the "has a relay but is also direct → not relayed" test pins the tricky quadrant of that truth table — nice.

One real issue inline (the flutter build ios --config-only call is cwd-dependent and pre-build actions don't run at the repo root); the rest are non-blocking nits.

— Caleb + Cache