feat: ambient scene system + unlock-screen banner (reduced-motion aware)

Reusable SceneWidget (QQuickWidget host) loads scene.qml — dusk sky, tiled
grass, running doge, bouncing WOW coins (slowed per feedback), drifting
money — and feeds it Config::reducedMotion so it freezes to a still when
that's set. First placement: a banner atop the unlock form. Scene rendered
& verified at banner + screensaver aspects via local qml6 harness. Assets
from wow-lite-wallet (friend's). Home banner / idle screensaver / per-tab
idle to follow on the same SceneWidget.

src/widgets/SceneWidget.cpp

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: BSD-3-Clause
+// SPDX-FileCopyrightText: The Monero Project
+
+#include "SceneWidget.h"
+
+#include <QQmlContext>
+
+#include "utils/config.h"
+
+SceneWidget::SceneWidget(QWidget *parent)
+    : QQuickWidget(parent)
+    , m_backend(new SceneBackend(this))
+{
+    m_backend->setReducedMotion(conf()->get(Config::reducedMotion).toBool());
+    this->rootContext()->setContextProperty("sceneCtx", m_backend);
+    this->setResizeMode(QQuickWidget::SizeRootObjectToView);
+    this->setSource(QUrl("qrc:/qml/scene.qml"));
+}
+
+void SceneWidget::refreshReducedMotion() {
+    m_backend->setReducedMotion(conf()->get(Config::reducedMotion).toBool());
+}