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>

docs/steamframe/PROGRESS.md

@@ -102,3 +102,23 @@
 - `MyTextField` opens the SteamVR keyboard on `activeFocus`. The template had to send the
   offscreen window an explicit `FocusIn`, otherwise `activeFocusItem` stayed null. The `qt6`
   branch doesn't send it. If text fields don't respond on the Frame, that is the cause (R4).
+
+### Step 4: manifest and installation (2026-10-07)
+
+**Changes:**
+- **`manifest.vrmanifest`:** `binary_path_linux` and `binary_path_linux_arm` →
+  `AdvancedSettings_app`.
+- **`manifest::handleManifests`:**
+  - `VR_Init(Utility)`; on `VRInitError_Init_NotAvailableToUtilityApps`,
+    `VR_Init(Background)`.
+  - Other errors: message and exit code `OPENVR_INIT_ERROR`, instead of
+    `initializeOpenVR`'s retry loop.
+- **`main.cpp`:** `--graphics-api` is checked before connecting to OpenVR. A wrong value
+  used to fail only after `initializeOpenVR`'s retry loop.
+
+**Results (x86_64, without SteamVR):**
+- `--force-install-manifest` exits right away with `OPENVR_INIT_ERROR` (−2).
+- `--graphics-api foo` exits right away with `GENERAL_FAILURE` (−1).
+- The build tree contains the binary, `libopenvr_api.so`, both manifests,
+  `default_action_manifests/` and `res/`.
+- Not tested yet: `Background` on the Frame (step 5).