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/manager/build.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python3
 import os
+import sys
 import subprocess
 from pathlib import Path
 
@@ -54,6 +55,21 @@ def build(spinner: Spinner, dirty: bool = False, minimal: bool = False) -> None:
 
     if scons.returncode == 0:
       Path('/data/openpilot/prebuilt').touch()
+
+      # CLEARPILOT: update prebuilt spinner if the new build is newer.
+      # Write to a temp path then os.replace so we can swap a binary that's
+      # currently executing (the in-process spinner holds the old one open).
+      new_spinner = Path(BASEDIR) / "selfdrive/ui/_spinner"
+      old_spinner = Path(BASEDIR) / "selfdrive/ui/qt/spinner"
+      if new_spinner.exists() and (not old_spinner.exists() or new_spinner.stat().st_mtime > old_spinner.stat().st_mtime):
+        import shutil
+        try:
+          tmp_spinner = old_spinner.with_name(old_spinner.name + ".new")
+          shutil.copy2(str(new_spinner), str(tmp_spinner))
+          os.replace(str(tmp_spinner), str(old_spinner))
+        except OSError as e:
+          print(f"CLP failed to update prebuilt spinner: {e}", file=sys.stderr)
+
       break
 
   if scons.returncode != 0:
@@ -69,8 +85,23 @@ def build(spinner: Spinner, dirty: bool = False, minimal: bool = False) -> None:
     # Show TextWindow
     spinner.close()
     if not os.getenv("CI"):
-      with TextWindow("openpilot failed to build\n \n" + error_s) as t:
-        t.wait_for_exit()
+      msg = "openpilot failed to build\n \n" + error_s
+      if os.getenv("BUILD_ONLY"):
+        # CLEARPILOT: BUILD_ONLY mode — spawn the text window fully detached
+        # (own session, /dev/null stdio) so it stays on screen after this
+        # script exits and doesn't hold our stdout/stderr pipes open.
+        print(error_s, file=sys.stderr)
+        devnull = open(os.devnull, 'r+b')
+        subprocess.Popen(
+          ["./text", msg],
+          cwd=os.path.join(BASEDIR, "selfdrive", "ui"),
+          stdin=devnull, stdout=devnull, stderr=devnull,
+          start_new_session=True,
+          close_fds=True,
+        )
+      else:
+        with TextWindow(msg) as t:
+          t.wait_for_exit()
     exit(1)
 
   # enforce max cache size