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

internal/app/network.go

@@ -0,0 +1,54 @@
+package app
+
+import (
+	"fmt"
+	"log"
+	"os/exec"
+	"strings"
+)
+
+// hasDHCPAddress reports whether iface holds an IPv4 address that ip(8) marks
+// as "dynamic" — the flag set on addresses with a finite valid_lft, which is
+// how DHCP-assigned leases appear (static addresses are "permanent").
+func hasDHCPAddress(iface string) (bool, error) {
+	out, err := exec.Command("ip", "-4", "-o", "addr", "show", "dev", iface).Output()
+	if err != nil {
+		return false, fmt.Errorf("ip addr show %s: %w", iface, err)
+	}
+	for _, line := range strings.Split(string(out), "\n") {
+		fields := strings.Fields(line)
+		hasInet := false
+		hasDynamic := false
+		for _, f := range fields {
+			switch f {
+			case "inet":
+				hasInet = true
+			case "dynamic":
+				hasDynamic = true
+			}
+		}
+		if hasInet && hasDynamic {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+// ensureUplink boots the WiFi uplink only if Ethernet is not already providing
+// connectivity: when iface has no DHCP-assigned IPv4, start wpa_supplicant so
+// the WiFi backend can take over.
+func ensureUplink(iface string) {
+	has, err := hasDHCPAddress(iface)
+	if err != nil {
+		log.Printf("Could not probe %s for DHCP address (%v); starting wpa_supplicant as fallback", iface, err)
+	} else if has {
+		log.Printf("DHCP address present on %s, leaving wpa_supplicant alone", iface)
+		return
+	} else {
+		log.Printf("No DHCP address on %s, starting wpa_supplicant", iface)
+	}
+
+	if out, err := exec.Command("systemctl", "start", "wpa_supplicant").CombinedOutput(); err != nil {
+		log.Printf("Failed to start wpa_supplicant: %v: %s", err, strings.TrimSpace(string(out)))
+	}
+}