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/quality.rs

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