Gaze mode is a mouse feature: drop its controller parts

Steam reads the Frame controllers itself, outside SteamVR's bindings, and every
press and release it sees takes SteamVR out of laser mode, so controller clicks
at the gaze can't be done cleanly (docs/gaze-controllers.md, from the gaze-first
branch's tests).

- Gaze precision and Gaze drag take a mouse button or a key combination only;
  the controller source, its aim steering, and POINTER_PRECISION_GAIN,
  POINTER_PRECISION_DEADZONE and POINTER_GAZE_DRAG_GAIN are gone.
- The gaze actions (including Gaze pointer on/off) can't be mapped to controller
  buttons: the relay ignores them there and doesn't ask the helper for those
  buttons, and Input Settings no longer offers them.
- Last used wins in gaze mode too: picking up a controller hands it the laser,
  as docs/design.md already said.
- The gaze dot shows all the time (POINTER_GAZE_DOT=always, the default;
  moving brings back the old behaviour), with a switch on the Gaze page.

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

input-settings/main.qml

@@ -833,12 +833,18 @@ Kirigami.ApplicationWindow {
                         }
                     }
                 }
+                Controls.Switch {
+                    Kirigami.FormData.label: "Gaze dot:"
+                    text: "Always shown (off: only while the mouse moves it)"
+                    checked: backend.gazeDotAlways
+                    onToggled: backend.setGazeDotAlways(checked)
+                }
                 Controls.Label {
                     Layout.maximumWidth: Kirigami.Units.gridUnit * 30
                     wrapMode: Text.WordWrap
-                    text: "Controllers: map a button to Gaze precision (hold, point the controller to steer, release to "
-                          + "click) or Gaze drag (the same, pressed at once) on the Controllers page. Gaze pointer on/off "
-                          + "can go on a controller button, a mouse button, or a key combination below."
+                    text: "Gaze works with the mouse. The Frame controllers don't take part, and moving one hands the "
+                          + "pointer back to the controllers. Gaze pointer on/off, Gaze precision, and Gaze drag can go on "
+                          + "a mouse button (Buttons page) or a key combination below."
                     opacity: 0.7
                     font: Kirigami.Theme.smallFont
                 }