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/package_files/manifest.vrmanifest

@@ -4,6 +4,8 @@
 		"app_key": "steam.overlay.1009850",
 		"launch_type": "binary",
 		"binary_path_windows": "AdvancedSettings.exe",
+		"binary_path_linux": "AdvancedSettings_app",
+		"binary_path_linux_arm": "AdvancedSettings_app",
 		"is_dashboard_overlay": true,
 
 		"strings": {