macOS: bundle the offscreen Qt platform plugin in the DMG

macdeployqt only deploys libqcocoa, so running QElectroTech with
QT_QPA_PLATFORM=offscreen aborts on the signed macOS bundle. This
prevents headless use (e.g. qet_mcp.py) without opening a Cocoa app.

Deploy platforms/libqoffscreen.dylib with macdeployqt -extra-plugins.
If it is not deployed, copy it from the Homebrew Qt and point its Qt
framework references to the bundle. The build stops if the plugin is
still missing. The existing signing loops already sign it with the
other plugins.

misc/MacQetDeploy_arm64_cmake.sh

@@ -149,7 +149,47 @@ if [ ! -d $BUNDLE ] ; then
     exit 1
 fi
 
-macdeployqt $BUNDLE
+macdeployqt $BUNDLE -extra-plugins=platforms/libqoffscreen.dylib
+
+### offscreen platform plugin (headless use, e.g. qet_mcp.py) ########
+# macdeployqt only deploys libqcocoa. Without libqoffscreen, running
+# QT_QPA_PLATFORM=offscreen aborts. If -extra-plugins above did not
+# deploy it, copy it from the Homebrew Qt and make it point to the
+# frameworks of the bundle. It is signed with the other plugins below.
+OFFSCREEN="$BUNDLE/Contents/PlugIns/platforms/libqoffscreen.dylib"
+if [ ! -f "$OFFSCREEN" ] ; then
+    echo "libqoffscreen.dylib not deployed by macdeployqt, copying it:"
+    SRC_OFFSCREEN=""
+    for d in "$(brew --prefix qt 2>/dev/null)/share/qt/plugins" \
+             "$(brew --prefix qt 2>/dev/null)/plugins" \
+             /opt/homebrew/share/qt/plugins ; do
+        if [ -f "$d/platforms/libqoffscreen.dylib" ] ; then
+            SRC_OFFSCREEN="$d/platforms/libqoffscreen.dylib"
+            break
+        fi
+    done
+    if [ -z "$SRC_OFFSCREEN" ] ; then
+        echo "ERROR: libqoffscreen.dylib not found in the Qt plugins"
+        exit 1
+    fi
+    mkdir -p "$BUNDLE/Contents/PlugIns/platforms"
+    cp -L "$SRC_OFFSCREEN" "$OFFSCREEN"
+    chmod u+w "$OFFSCREEN"
+    # Qt frameworks: /opt/homebrew/.../QtGui.framework/Versions/A/QtGui
+    # -> inside the bundle
+    otool -L "$OFFSCREEN" | awk 'NR>1 { print $1 }' \
+        | grep -E '/Qt[A-Za-z0-9]+\.framework/' | while read ref; do
+        fwname=$(echo "$ref" | sed -E 's#.*/(Qt[A-Za-z0-9]+)\.framework/.*#\1#')
+        install_name_tool -change "$ref" \
+            "@executable_path/../Frameworks/$fwname.framework/Versions/A/$fwname" \
+            "$OFFSCREEN"
+        echo "  Fixed ref: $fwname"
+    done
+fi
+if [ ! -f "$OFFSCREEN" ] ; then
+    echo "ERROR: libqoffscreen.dylib missing from the bundle"
+    exit 1
+fi
 
 ### fix Homebrew dependencies macdeployqt could not handle ##########
 # Recent Homebrew bottles (brotli, webp, sharpyuv...) reference their own