Manifest for Linux; connect as Background app where Utility is refused

- manifest.vrmanifest: binary_path_linux and binary_path_linux_arm. The
  Steam Frame's SteamVR reads only binary_path_linux_arm.
- handleManifests(): connect as Utility app first (works without a running
  SteamVR, e.g. from the Windows installer); if SteamVR refuses Utility apps,
  as on the Steam Frame, connect as Background app. Fail right away on other
  errors instead of going through initializeOpenVR()'s retry loop.
- main(): check --graphics-api before connecting to OpenVR, so a wrong value
  fails right away.

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

src/main.cpp

@@ -45,14 +45,8 @@ int main( int argc, char* argv[] )
                                    commandLineArgs.forceRemoveManifest );
     }
 
-    openvr_init::initializeOpenVR(
-        openvr_init::OpenVrInitializationType::Overlay );
-
-    // Render backend (render/RenderBackend.h). Created after OpenVR, because
-    // the Vulkan backend needs SteamVR's device requirements
-    // (openvr_init::renderInterop()), and before OverlayController, because
-    // QQuickWindow::setGraphicsApi() must run before any QQuickWindow exists
-    // (OverlayController owns one).
+    // Render backend (render/RenderBackend.h). Chosen before connecting to
+    // OpenVR, so a wrong --graphics-api fails right away.
     GraphicsApi graphicsApi = GraphicsApi::OpenGL;
     if ( !parseGraphicsApi( commandLineArgs.graphicsApi, &graphicsApi ) )
     {
@@ -68,6 +62,14 @@ int main( int argc, char* argv[] )
                     << "is not compiled into this build";
         return ReturnErrorCode::GENERAL_FAILURE;
     }
+
+    openvr_init::initializeOpenVR(
+        openvr_init::OpenVrInitializationType::Overlay );
+
+    // Initialized after OpenVR, because the Vulkan backend needs SteamVR's
+    // device requirements (openvr_init::renderInterop()), and before
+    // OverlayController, because QQuickWindow::setGraphicsApi() must run
+    // before any QQuickWindow exists (OverlayController owns one).
     QString renderBackendError;
     if ( !renderBackend->initialize( openvr_init::renderInterop(),
                                      &renderBackendError ) )