Add head follow: the pointer comes along when you turn your head

Off by default. With POINTER_FOLLOW=1 (a switch on the Pointer page of
Frametop Input Settings, or a mouse button mapped to Head follow on/off),
the cursor rides on a reference direction leashed POINTER_LEASH_DEG (10)
from where you face. Within the leash it stays put in the room; past it,
it turns with your head and keeps its offset. A leash of 0 locks it to
your view. Head roll is ignored, the cursor stays within 40 degrees of
the reference, and it holds still in the room while the left button is
down so the head can't nudge a click or a drag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

input-settings/main.qml

@@ -314,6 +314,12 @@ Kirigami.ApplicationWindow {
             ]
 
             Kirigami.FormLayout {
+                Controls.Switch {
+                    Kirigami.FormData.label: "Head follow:"
+                    text: "Pointer follows your head (leash below; 0° locks it to your view)"
+                    checked: backend.pointerFollow
+                    onToggled: backend.setPointerFollow(checked)
+                }
                 Repeater {
                     model: backend.pointerSettings
                     delegate: RowLayout {