fix(network): derive the host IP from the default route, not hostname -I order

On a fresh ISO node, NetBird's own WireGuard tunnel (10.44.0.1) sorted
ahead of the real NIC in hostname -I, so the NetBird launch URL (and the
{{HOST_IP}} baked into its cert/config) pointed at the tunnel instead of
the LAN address. The main routing table's default route names the
physical uplink even while a VPN is active (NetBird/Tailscale steer
traffic via policy-routing rules, not the main table), so read src/dev
from 'ip -4 route show default' first, then fall back to a connected UDP
socket's source address, then to the old hostname -I scan.

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

core/archipelago/src/config.rs

@@ -80,19 +80,11 @@ pub struct Config {
 }
 
 impl Config {
-    /// Detect primary host IP (first non-loopback IPv4)
+    /// Detect primary host IP (default-route interface, not `hostname -I` order)
     async fn detect_host_ip() -> Result<String> {
-        let output = tokio::process::Command::new("hostname")
-            .args(["-I"])
-            .output()
+        Ok(crate::host_ip::primary_host_ipv4()
             .await
-            .context("Failed to run hostname -I")?;
-        let s = String::from_utf8_lossy(&output.stdout);
-        let ip = s
-            .split_whitespace()
-            .find(|s| !s.starts_with("127.") && s.contains('.'))
-            .unwrap_or("127.0.0.1");
-        Ok(ip.to_string())
+            .unwrap_or_else(|| "127.0.0.1".to_string()))
     }
 
     pub async fn load() -> Result<Self> {