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.

selfdrive/ui/qt/home.cc

@@ -10,8 +10,6 @@
 #include "selfdrive/ui/qt/widgets/drive_stats.h"
 #include "system/hardware/hw.h"
 
-#include <QWebEngineView>
-
 // HomeWindow: the container for the offroad and onroad UIs
 
 HomeWindow::HomeWindow(QWidget* parent) : QWidget(parent) {

selfdrive/ui/qt/onroad.cc

@@ -78,7 +78,7 @@ void OnroadWindow::updateState(const UIState &s) {
 
   QColor bgColor = bg_colors[s.status];
   
-  if (paramsMemory.getBool("no_lat_lane_change") == 1 || screenDisplayMode == 2) {
+  if (paramsMemory.getBool("no_lat_lane_change") == 1 || nvg->screenDisplayMode == 2) {
     bgColor = bg_colors[STATUS_DISENGAGED];
   }