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/ReceiveWidget.cpp

@@ -23,7 +23,7 @@ ReceiveWidget::ReceiveWidget(Wallet *wallet, QWidget *parent)
 
     // wowlet: ambient scene as a short banner under the address table. A layout
     // item, never an overlay over the table.
-    auto *scene = new SceneWidget(this);
+    auto *scene = new SceneWidget(this, SceneWidget::Dawn);
     scene->setMinimumHeight(110);
     scene->setMaximumHeight(130);
     ui->verticalLayout->addWidget(scene);