keep default theme objects alive for the process lifetime

[wehrwolfmann]

The default MdColorMgr / ThemeSize / PageContext that MProp hands out live in a
Q_GLOBAL_STATIC in src/style/theme.cpp. When a process reaches static destruction while a
QQmlEngine is still alive, those objects go away first, and every binding reading
MD.MProp.color or MD.Token.color re-evaluates against a destroyed QObject and prints
TypeError: Cannot read property '...' of null. Token.qml:36 (isDarkTheme) is in that list
too, so the singleton trips over it as well.

It is not an exotic ordering: any engine that outlives main() hits it — a heap engine that is
never deleted, exit() from a watchdog or crash path, or an engine whose lifetime belongs to a
scripting host. In an app embedding the library, one killed run printed 191 of these lines.

Reproducer (v0.3.0, Qt 6.11.2, Xvfb + software GL): an MD.Page holding 20 rows of
MD.Divider + MD.Badge + MD.ScrollBar + MD.Text in a plain QQuickView, hosted by

auto* view = new QQuickView();   // the engine outlives main()
view->setSource(scene); view->show();
QTimer::singleShot(1500, &app, []{ QCoreApplication::quit(); });
app.exec();
std::exit(0);                    // statics are destroyed with the engine still up

Before the patch: 101 TypeError lines. After: 0. Same scene, otherwise the same binary.

The change creates the defaults once and keeps them for the process lifetime. Nothing moves while
the engine is up: colors read through an attached provider and through the default are identical
before and after (16 roles plus mode, isDarkTheme, themeMode, inherited elevation /
textColor, and reset fallback); the only difference in the dump is the printed address, now
heap instead of static storage. ctest is 42/42 both ways, and example starts clean.

I left Token.qml and the raw pointers in AttachProp untouched on purpose — guarding the QML
would only hide the symptom, and turning the attached property into a QPointer is a larger
decision than this defect calls for. Glad to go that way instead if you prefer it.