Copy Qt platforms/qwindows.dll plugin alongside executable

tools/chatgui/build.sh

@@ -64,6 +64,11 @@ else
         todo="$next_todo"
     done
 fi
+# Qt platform plugin (loaded dynamically, not in import table)
+mkdir -p "$BUILD_DIR/platforms"
+for d in /ucrt64/share/qt5/plugins /ucrt64/lib/qt5/plugins; do
+    cp "$d/platforms/qwindows.dll" "$BUILD_DIR/platforms/" 2>/dev/null && break
+done
 cp "$SRC_DIR/rlottie_build/librlottie.dll" "$BUILD_DIR/" 2>/dev/null && log "  OK: librlottie.dll" || log "  WARN: librlottie.dll not found"
 
 log ""