make baseline build & launch cleanly

- onroad.cc: fix screenDisplayMode reference to nvg->screenDisplayMode
  (baseline had a stale reference to the variable after it moved into
  AnnotatedCameraWidget). Restores the original conditional intent.
- ui/SConscript + home.cc: drop QtWebEngine include/link entirely
  (no longer used by any active code).
- build.py: BUILD_ONLY env var spawns the failure TextWindow fully
  detached (own session, /dev/null stdio) so build_only.sh exits and
  caller can capture stderr; spinner binary update uses temp+os.replace
  so a running spinner doesn't ETXTBSY the build.
- build_only.sh: tee build output to /tmp/build.log and propagate
  build.py's exit code via PIPESTATUS.

Verified: build_only.sh completes cleanly, launch_openpilot.sh boots
the manager and spawns the standard process set.

build_only.sh

@@ -35,4 +35,6 @@ source "$BASEDIR/build_preflight.sh"
 cd "$BASEDIR/selfdrive/manager"
 rm -f "$BASEDIR/prebuilt"
 
-BUILD_ONLY=1 exec ./build.py
+# Tee output to /tmp/build.log for inspection after the script exits
+BUILD_ONLY=1 ./build.py 2>&1 | tee /tmp/build.log
+exit "${PIPESTATUS[0]}"