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>

launch_chffrplus.sh

@@ -85,6 +85,9 @@ function launch {
   # write tmux scrollback to a file
   tmux capture-pane -pq -S-1000 > /tmp/launch_log
 
+  # Preflight: create dirs git can't track
+  source "$DIR/build_preflight.sh"
+
   # start manager
   cd selfdrive/manager
   if [ ! -f $DIR/prebuilt ]; then