WiFi screen: reach the root helper through a socket instead of sudo; adapter dropdown

The kiosk app can end up with the no-new-privileges flag, after which sudo
refuses to run ("The no new privileges flag is set"). The helper is now
served over a root-owned, kiosk-group-only unix socket (systemd socket
activation); sudo stays as a fallback. With several WiFi adapters the screen
shows a dropdown and lists all of them; status/scan/connect take an iface.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013wi3ZQveYdjfcrEsyazq32

provision/files/etc/systemd/system/kiosk-wifi-helper.socket

@@ -0,0 +1,14 @@
+[Unit]
+Description=Kiosk WiFi helper socket (used by the kiosk app's WiFi screen)
+
+[Socket]
+ListenStream=/run/kiosk-wifi-helper.sock
+SocketUser=root
+# Replaced with the kiosk user's group when installed (lib/provision.sh).
+SocketGroup=@KIOSK_GROUP@
+SocketMode=0660
+Accept=yes
+MaxConnections=4
+
+[Install]
+WantedBy=sockets.target

provision/files/etc/systemd/system/kiosk-wifi-helper@.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Kiosk WiFi helper (one request)
+
+[Service]
+ExecStart=/usr/local/bin/kiosk-wifi-helper-socket
+StandardInput=socket
+StandardOutput=socket
+StandardError=journal
+# A connect waits up to ~30s for an address, plus netplan; never run forever.
+RuntimeMaxSec=180