fix fresh-install build: add preflight script, restore third_party binaries, remove WebEngine dep

- Add build_preflight.sh to create obj/ dirs that git can't track (body/board/obj, panda/board/obj)
- Wire preflight into build_only.sh and launch_chffrplus.sh
- Restore missing third_party binaries (libyuv, snpe, acados, maplibre) that were text pointers
- Remove dead Qt5WebEngineWidgets dependency from UI SConscript

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

build_only.sh

@@ -29,6 +29,9 @@ export PYTHONPATH="$BASEDIR"
 sudo chgrp gpu /dev/adsprpc-smd /dev/ion /dev/kgsl-3d0 2>/dev/null
 sudo chmod 660 /dev/adsprpc-smd /dev/ion /dev/kgsl-3d0 2>/dev/null
 
+# Preflight: create dirs git can't track
+source "$BASEDIR/build_preflight.sh"
+
 cd "$BASEDIR/selfdrive/manager"
 rm -f "$BASEDIR/prebuilt"