fix(companion): pairing QR advertises the LAN address, never a tailnet IP

The QR mirrored the browser origin — an operator browsing over Tailscale
handed the phone a 100.x address it has no route to, so pairing silently
never connected (reported 2026-07-22; the 'random password' alongside it
was the device token working as designed). system.get-hostname now also
returns the default-route LAN IPv4; the QR substitutes it (or the mDNS
name) whenever the origin is localhost or a CGNAT/tailnet 100.64/10 IP.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/api/rpc/system/handlers.rs

@@ -139,9 +139,17 @@ impl RpcHandler {
             .await
             .map(|s| s.trim().to_string())
             .unwrap_or_else(|_| "archipelago".to_string());
+        // LAN IPv4 rides along for the companion pairing QR: when the
+        // operator's browser reaches this node over Tailscale/VPN or
+        // localhost, that origin is useless to a phone on the LAN — the QR
+        // must advertise an address the phone can actually dial
+        // (2026-07-22: a pairing QR carried a tailnet 100.x IP and the
+        // companion could never connect).
+        let lan_ip = crate::host_ip::primary_host_ipv4().await;
         Ok(serde_json::json!({
             "hostname": hostname,
             "mdns_hostname": format!("{hostname}.local"),
+            "lan_ip": lan_ip,
         }))
     }