A one-line installer, and gaze mode in install.sh

- get.sh: curl ... | bash asks for stable (main) or experimental,
  clones or updates ~/frametop, and runs install.sh; run it again to
  update or switch
- install.sh offers gaze mode (step 8, yes by default) and notes what
  to do if an SSH connection drops
- Input Settings' Gaze page says when the gaze service isn't installed

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

input-settings/main.qml

@@ -1018,7 +1018,8 @@ Kirigami.ApplicationWindow {
                     Kirigami.FormData.label: "Service:"
                     text: backend.gazeServiceRunning
                           ? (gpage.status.ft_gaze ? "running" : "running, eye tracker reader restarting")
-                          : "not running (frametop-gaze.service, starts with SteamVR)"
+                          : backend.gazeServiceInstalled ? "not running (frametop-gaze.service, starts with SteamVR)"
+                          : "not installed: run gaze/run.sh install in the Frametop folder, in a terminal"
                     color: backend.gazeServiceRunning ? Kirigami.Theme.textColor : Kirigami.Theme.negativeTextColor
                 }
                 Controls.Label {