viewport: wire up the backface-culling setting

The "Backface Culling" checkbox persisted a value and reflected it, but
nothing consumed AppSettings::backfaceCulling — the opaque pipeline
hardcoded cullMode = Back, so toggling had no effect.

Build a second opaque pipeline (cullMode None) alongside the culled one and
pick between them per-frame from a backface_culling_ flag; setBackfaceCulling
flips the flag and requests a redraw (no rebuild). ViewportWindow forwards
it, and MainWindow applies the persisted value at startup and re-applies on
change — same wiring as the nav preset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.cpp

@@ -1499,6 +1499,10 @@ void ViewportWindow::applyNavPreset(const char* name) {
     select_button_ = toQtBtn(b.select);  select_mods_ = toQtMod(b.select_mod);
 }
 
+void ViewportWindow::setBackfaceCulling(bool enabled) {
+    core_.setBackfaceCulling(enabled);
+}
+
 // -----------------------------------------------------------------------------
 // One-shot framebuffer capture → PNG
 // -----------------------------------------------------------------------------