Allow custom RelayMode via XS_RELAY_URL env var

[joeblew999]

Why

xs currently hardcodes iroh::RelayMode::Default in three places (listener.rs:303, listener.rs:374, client/connect.rs:74). RelayMode::Default routes via n0's public relay infrastructure (use1, euw1, apse-1).

That's the right default, but operators have legitimate reasons to override:

• Regions where n0's defaults aren't reachable. As of writing, apse-1.relay.iroh.network has no public DNS A record. Iroh nodes in Asia hang indefinitely on home_relay().initialized().await — a laptop in Thailand cannot bind an iroh:// endpoint at all. Self-hosting an iroh-relay (e.g. on a Hetzner Singapore VM) is the workaround, but today it requires source-patching xs.
• Compliance / air-gapped networks where outbound to relay.iroh.network isn't permitted.
• Local development against a self-hosted dev relay.

What

Introduces src/relay.rs exporting relay_mode_from_env() -> RelayMode:

XS_RELAY_URL value	resolves to
empty / unset	RelayMode::Default (preserves existing behavior — strict superset)
"disabled" (case-insensitive)	RelayMode::Disabled
any other value	parsed as iroh_base::RelayUrl → RelayMode::Custom(RelayMap)
parse failure	falls back to RelayMode::Default with a tracing::warn!

The three iroh endpoint builders in listener.rs and client/connect.rs now call this helper instead of using the hardcoded constant.

Backward compatibility

Strict superset. Users who don't set XS_RELAY_URL get exactly the prior behavior.

Tests

6 unit tests in src/relay::tests cover: unset, empty, whitespace, "DISABLED" keyword, valid URL → Custom, malformed URL → Default fallback.

All existing tests pass (cargo test --lib).

Verification

Built locally and run from a Thai laptop against a self-hosted iroh-relay 1.0.0-rc.0 on Hetzner Singapore (5.223.94.174 in --dev mode):

XS_RELAY_URL=http://5.223.94.174:3340 ./target/release/xs serve /tmp/store --expose iroh://

09:54:38.859  INFO XS_RELAY_URL=http://5.223.94.174:3340 — using RelayMode::Custom
09:54:38.888 DEBUG Iroh endpoint bound successfully
09:54:41.892  INFO Iroh endpoint ready with node ID: f19a07801049…
09:54:41.892  INFO Iroh ticket: nodeadyzub4ac…

Without the patch, the bind hangs forever in that environment.

Naming alternative

Happy to rename the env var (e.g. XS_RELAY or CROSSTREAM_RELAY_URL) if there's a project convention I should follow.