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)

pointer/helper/ft-pointer.cpp

@@ -567,7 +567,11 @@ int main() {
 
     vr::EVRInitError err = vr::VRInitError_None;
     while (true) {
-        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) break;
         std::fprintf(stderr, "waiting for SteamVR: %s\n", vr::VR_GetVRInitErrorAsEnglishDescription(err));
         std::this_thread::sleep_for(std::chrono::seconds(2));