feat(scene): per-lap variety, the occasional face-plant, and a sky per tab

Every tab was running the identical eight seconds forever. Three changes:

- Each lap rolls its own timing: 3-5s of empty field, and the money now enters
  on its own delay at its own pace instead of shadowing the doge every time.
- Roughly one lap in six the doge eats it partway down the field: he tips
  nose-first onto the grass, GAME OVER comes up, and the coins he never reached
  are still sitting there. Costs no new art — it is the running sprite pivoted
  on its own feet.
- A `variant` on SceneWidget picks the sky: dusk (History, lock screen), dawn
  (Receive), night (Send, with stars and a moon instead of the sun).

The cycle is now a timer-driven state machine rather than one SequentialAnimation.
That is not stylistic: a lap that rolls its own durations has to assign them
somewhere, and assigning a duration to a child of a *running* SequentialAnimation
makes the group recompute and re-enter its own ScriptAction until the JS stack
blows. Qt flags the binding form of the same mistake as a binding loop.

Verified by headless qml6 render: a forced trip lap, and a clean run with no QML
warnings.

src/widgets/SceneWidget.cpp

@@ -7,11 +7,12 @@
 
 #include "utils/config.h"
 
-SceneWidget::SceneWidget(QWidget *parent)
+SceneWidget::SceneWidget(QWidget *parent, Variant variant)
     : QQuickWidget(parent)
     , m_backend(new SceneBackend(this))
 {
     m_backend->setReducedMotion(conf()->get(Config::reducedMotion).toBool());
+    m_backend->setVariant(static_cast<int>(variant));
     this->rootContext()->setContextProperty("sceneCtx", m_backend);
     this->setResizeMode(QQuickWidget::SizeRootObjectToView);
     this->setSource(QUrl("qrc:/qml/scene.qml"));