feat(relay): add --relay / PIXELPASS_RELAY override

Both host and viewer hardcoded presets::N0, pinning every session to the
bundled relays (which on iroh rc.0 are the canary-grade defaults). Add a
shared common::endpoint::bind() that keeps N0's DNS discovery + crypto but
swaps in a RelayMode::Custom single-relay map when --relay (or the
PIXELPASS_RELAY env var, so GUI children inherit it) is set.

Lets users point at a self-hosted relay or staging today; the production
relays (*.relay.iroh.network) speak a newer protocol that rc.0 rejects
("invalid iroh-relay version header"), so they only become usable — and
the default — after an iroh GA bump. Verified: override connects cleanly
through staging; bad URLs are rejected before any network work.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/host/mod.rs

@@ -7,7 +7,7 @@ mod wayland;
 mod x11;
 
 use anyhow::{Result, bail};
-use iroh::endpoint::{Connection, presets};
+use iroh::endpoint::Connection;
 use iroh::{Endpoint, EndpointAddr};
 use iroh_tickets::endpoint::EndpointTicket;
 use std::collections::HashMap;
@@ -17,7 +17,7 @@ use tokio_util::sync::CancellationToken;
 
 use crate::cli::HostOpts;
 use crate::common::{
-    alpn::ALPN, bandwidth, config, config::BandwidthStatus, deps, display::DisplayServer, output,
+    bandwidth, config, config::BandwidthStatus, deps, display::DisplayServer, endpoint, output,
     signal, tunnel,
 };
 
@@ -74,10 +74,7 @@ pub async fn run(opts: HostOpts) -> Result<()> {
 
     let cancel = signal::install_ctrl_c();
 
-    let endpoint = Endpoint::builder(presets::N0)
-        .alpns(vec![ALPN.to_vec()])
-        .bind()
-        .await?;
+    let endpoint = endpoint::bind(opts.relay.as_deref()).await?;
 
     // Relay-only ticket: wait for the home relay to connect, then keep only
     // the endpoint id + relay URL and drop the direct IP candidates. The relay

src/host/quality.rs

@@ -189,6 +189,7 @@ mod tests {
             no_hwencode: false,
             max_viewers,
             interactive: false,
+            relay: None,
         }
     }