Start Frametop's SteamVR clients only once SteamVR is up (#6)

The pointer and gaze services need steamvr.service to be running (Requisite=), and ft-pointer, ft-screens, and ft-gaze connect as a background app before switching to overlay, so they never start a vrserver of their own. One started from the dev container never finds the headset, which left a reboot stuck in a loop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
(cherry picked from commit 51b4e79318)

screens/vr.cpp

@@ -1148,7 +1148,11 @@ extern "C" {
 
 bool ft_vr_init(void) {
     vr::EVRInitError err = vr::VRInitError_None;
-    vr::VR_Init(&err, vr::VRApplication_Overlay);
+    vr::VR_Init(&err, vr::VRApplication_Background);
+    if (err == vr::VRInitError_None) {
+        vr::VR_Shutdown();
+        vr::VR_Init(&err, vr::VRApplication_Overlay);
+    }
     if (err != vr::VRInitError_None) {
         std::fprintf(stderr, "openvr: %s\n", vr::VR_GetVRInitErrorAsEnglishDescription(err));
         return false;