Auto-detect network interface when not configured

If no interface is set in the config, the ARP scanner now picks the
first non-loopback, up, IPv4-enabled interface automatically. The
setting is optional in the TOML config, sample config, and Nix module.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/settings.rs

@@ -13,7 +13,7 @@ pub struct Database {
 
 #[derive(Debug, Deserialize, Clone)]
 pub struct Networking {
-    pub interface: String,
+    pub interface: Option<String>,
 }
 
 #[derive(Debug, Deserialize, Clone)]