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_preflight.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/bash
+# CLEARPILOT: build preflight — create directories and fix state that
+# git cannot track but the build requires.  Called by build_only.sh and
+# launch_chffrplus.sh before scons runs.
+
+BASEDIR="${BASEDIR:-/data/openpilot}"
+
+# SConscript files write generated headers into obj/ directories at
+# parse time — these must exist before scons starts.
+mkdir -p "$BASEDIR/body/board/obj"
+mkdir -p "$BASEDIR/panda/board/obj"