app: Surface Ethernet uplink in the UI and gate wpa_supplicant access

When the configured Ethernet interface holds a DHCP-assigned IPv4 at
startup, the app now skips wifi.Initialize / StartEventMonitoring and
guards every wifi.* wrapper against a nil backend. This prevents D-Bus
calls to fi.w1.wpa_supplicant1 from re-activating the daemon via
dbus-activation, honoring the "do nothing" intent of the Ethernet path.

The probed state is exposed in SystemStatus and rendered in the header
as a third pill ("Ethernet · <IP>"); a new "disabled" connectionState
covers the WiFi pill in this mode.

internal/wifi/wifi.go

@@ -1,6 +1,7 @@
 package wifi
 
 import (
+	"errors"
 	"fmt"
 	"sort"
 	"strings"
@@ -16,6 +17,13 @@ var (
 	wifiBroadcaster *WifiBroadcaster
 )
 
+// errWifiDisabled is returned by every wifi.* wrapper when no backend has
+// been initialized. This happens when the application chose not to start
+// wpa_supplicant because the Ethernet uplink is already providing
+// connectivity — touching the D-Bus interface in that mode would re-activate
+// the daemon via dbus-activation, defeating the intent.
+var errWifiDisabled = errors.New("wifi backend disabled (Ethernet uplink active)")
+
 // Initialize initializes the WiFi service with the specified backend
 func Initialize(interfaceName string, backendName string) error {
 	// Create the appropriate backend using the factory
@@ -38,6 +46,9 @@ func Close() {
 
 // GetCachedNetworks returns previously discovered networks without triggering a scan
 func GetCachedNetworks() ([]models.WiFiNetwork, error) {
+	if wifiBackend == nil {
+		return nil, errWifiDisabled
+	}
 	// Get ordered networks from backend
 	backendNetworks, err := wifiBackend.GetOrderedNetworks()
 	if err != nil {
@@ -67,6 +78,9 @@ func GetCachedNetworks() ([]models.WiFiNetwork, error) {
 
 // ScanNetworks scans for available WiFi networks
 func ScanNetworks() ([]models.WiFiNetwork, error) {
+	if wifiBackend == nil {
+		return nil, errWifiDisabled
+	}
 	// Check if already scanning
 	scanning, err := wifiBackend.IsScanning()
 	if err == nil && scanning {
@@ -114,6 +128,9 @@ func ScanNetworks() ([]models.WiFiNetwork, error) {
 
 // Connect connects to a WiFi network
 func Connect(ssid, password string) error {
+	if wifiBackend == nil {
+		return errWifiDisabled
+	}
 	// Use backend to connect
 	if err := wifiBackend.Connect(ssid, password); err != nil {
 		return err
@@ -132,11 +149,17 @@ func Connect(ssid, password string) error {
 
 // Disconnect disconnects from the current WiFi network
 func Disconnect() error {
+	if wifiBackend == nil {
+		return errWifiDisabled
+	}
 	return wifiBackend.Disconnect()
 }
 
 // IsConnected checks if WiFi is connected
 func IsConnected() bool {
+	if wifiBackend == nil {
+		return false
+	}
 	state, err := wifiBackend.GetConnectionState()
 	if err != nil {
 		return false
@@ -146,11 +169,17 @@ func IsConnected() bool {
 
 // GetConnectedSSID returns the SSID of the currently connected network
 func GetConnectedSSID() string {
+	if wifiBackend == nil {
+		return ""
+	}
 	return wifiBackend.GetConnectedSSID()
 }
 
 // GetConnectionState returns the current WiFi connection state
 func GetConnectionState() string {
+	if wifiBackend == nil {
+		return string(backend.StateDisconnected)
+	}
 	state, err := wifiBackend.GetConnectionState()
 	if err != nil {
 		return string(backend.StateDisconnected)
@@ -160,6 +189,9 @@ func GetConnectionState() string {
 
 // StartEventMonitoring initializes signal monitoring and WebSocket broadcasting
 func StartEventMonitoring() error {
+	if wifiBackend == nil {
+		return nil
+	}
 	// Initialize broadcaster
 	wifiBroadcaster = NewWifiBroadcaster()