Let Ethernet be switched off and on

Wi-Fi had a switch and wired did not, which left no way to take the cable down
without reaching for nmcli -- and this machine has two interfaces on one subnet,
so turning one off is a genuinely useful thing to be able to do.

Both halves are needed to turn it off. Disconnecting alone survives the session
but not a carrier event or a reboot, because NetworkManager brings an
autoconnecting device straight back; the switch sets the device's autoconnect
alongside it. The device property rather than the connection profile, so a
toggle here does not quietly rewrite a saved connection somebody expects to come
up at boot.

The first version built a trap door. It decided whether the switch was usable
from hasLink, which reads false while a device is merely disconnected even
though NetworkManager still reports the carrier as on -- so turning Ethernet off
made the switch disable itself, blame the cable, and offer no way back. A wired
device that exists can always be asked to come up; if there is really no cable
the attempt fails and says so, which is the honest failure. The word "off" is
used where the old text guessed "no cable", because nothing available here can
tell those apart.

Verified as a round trip against the real device, including that off stays off
through eight seconds rather than reconnecting a moment later, and that the
mechanism tested is the one the code uses -- the first test drove the connection
profile while the code drives the device, which are different things.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/modules/quicksettings/QuickSettingsPanel.qml

@@ -107,6 +107,27 @@ Item {
                 onExpanded: root.expand("wifi")
             }
 
+            // Only present when there is a wired device at all -- most machines
+            // this runs on have one, but a laptop without a dock does not, and
+            // an Ethernet tile there would be a control for absent hardware.
+            Toggle {
+                width: root.cellWidth
+                visible: Connectivity.wiredDevice !== null
+                icon: Connectivity.wiredOn
+                    ? "network-wired-symbolic"
+                    : "network-wired-disconnected-symbolic"
+                label: "Ethernet"
+                active: Connectivity.wiredOn
+                enabled: Connectivity.wiredAvailable
+                sublabel: {
+                    if (Connectivity.wiredOn)
+                        return Connectivity.wiredDevice.linkSpeed > 0
+                            ? Connectivity.wiredDevice.linkSpeed + " Mb/s" : "Connected";
+                    return "Off";
+                }
+                onToggled: Connectivity.setWired(!Connectivity.wiredOn)
+            }
+
             Toggle {
                 width: root.cellWidth
                 icon: root.btAdapter && root.btAdapter.enabled ? "bluetooth-active-symbolic" : "bluetooth-disabled-symbolic"