app: Start wpa_supplicant only when Ethernet has no DHCP lease

Probe the configured Ethernet interface (default eth0, overridable via
-ethernet-interface) at startup. If no DHCP-assigned IPv4 is present,
start the wpa_supplicant service so the WiFi backend has something to
talk to; otherwise leave it alone and rely on the wired uplink.

internal/app/app.go

@@ -57,6 +57,11 @@ func (a *App) Initialize(cfg *config.Config) error {
 	// Store config reference
 	a.Config = cfg
 
+	// If Ethernet uplink is not already providing connectivity (no DHCP lease
+	// on the configured interface), bring up wpa_supplicant so the WiFi
+	// backend has something to talk to.
+	ensureUplink(cfg.EthernetInterface)
+
 	// Initialize WiFi backend
 	if err := wifi.Initialize(cfg.WifiInterface, cfg.WifiBackend); err != nil {
 		return err