Make the rail reset zero the sensor in live and the display in replay

The replay view disabled a control whose display reset is exactly what an
operator wants there, and the bare "Reset" hid that the live one zeroes the
sensor tracker; the label, tooltip and status line now name the target, and
Ctrl+R follows the view. Verified with the studio suite, ui_check (628
checks) and real-GL rail snapshots of both views.

apps/studio/docs/ui-elements.json

@@ -119,7 +119,7 @@
         { "id": "stop", "path": "toolbar.btn_stop", "objectName": null, "style": "dangerOutline / dangerButton", "class": "QPushButton",
           "purpose": "end the live session, leave the replay, or cancel a connect that has not returned" },
         { "id": "reset", "path": "toolbar.btn_reset", "objectName": null, "class": "QPushButton",
-          "purpose": "zero the device tracker; enabled by the session state, never by which panel is on screen" },
+          "purpose": "one control, one meaning per view: in the live view it zeroes the sensor tracker (enabled by the session state, never by which panel is on screen, and the display baseline restarts with it); in the replay view, which owns no device, it resets the display" },
         { "id": "settings_toggle", "path": "toolbar.btn_settings", "objectName": null, "style": "ghostButton", "class": "QPushButton",
           "purpose": "expand or collapse the settings area" }
       ]
@@ -201,7 +201,7 @@
         { "id": "act_preview", "path": "act_preview", "objectName": null, "class": "QAction",
           "purpose": "F6: camera preview on/off" },
         { "id": "act_reset", "path": "act_reset", "objectName": null, "class": "QAction",
-          "purpose": "Ctrl+R: zero the device tracker, the same action as the rail's reset" },
+          "purpose": "Ctrl+R: the rail's reset on a second surface, named from the view it is in (reset sensor tracker while measuring, reset display while replaying)" },
         { "id": "act_reset_view", "path": "act_reset_view", "objectName": null, "class": "QAction",
           "purpose": "F8: reset the views (curves, window, cursor, 3D camera) without touching the device" },
         { "id": "act_pause", "path": "act_pause", "objectName": null, "class": "QAction",