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.

openapi.yaml

@@ -369,6 +369,26 @@ components:
         - ssid
         - password
 
+    EthernetStatus:
+      type: object
+      description: State of the wired uplink interface (probed via ip(8))
+      properties:
+        active:
+          type: boolean
+          description: True when the interface holds a DHCP-assigned IPv4
+          example: true
+        interface:
+          type: string
+          description: Probed interface name
+          example: "eth0"
+        ipv4:
+          type: string
+          description: DHCP-assigned IPv4 address (empty when no lease)
+          example: "192.168.1.42"
+      required:
+        - active
+        - interface
+
     HotspotStatus:
       type: object
       description: Detailed hotspot status from hostapd_cli
@@ -480,13 +500,14 @@ components:
           example: true
         connectionState:
           type: string
-          description: Current WiFi connection state
+          description: Current WiFi connection state ("disabled" when Ethernet uplink is active and the WiFi backend is intentionally not initialized)
           enum:
             - connected
             - disconnected
             - connecting
             - disconnecting
             - roaming
+            - disabled
           example: "connected"
         connectedSSID:
           type: string
@@ -497,6 +518,11 @@ components:
             - $ref: '#/components/schemas/HotspotStatus'
           nullable: true
           description: Detailed hotspot status (null if hotspot is not running)
+        ethernetStatus:
+          allOf:
+            - $ref: '#/components/schemas/EthernetStatus'
+          nullable: true
+          description: Wired uplink state (null when not yet probed)
         connectedCount:
           type: integer
           description: Number of devices connected to hotspot