perf: smooth UI animations and reduce repaint overhead

Batch of performance and animation work: shared FadeHelper/MotionPrefs
primitives, hover animators, shadow pixmap caching, smooth scrolling,
seek time bubble, and widespread component repaint reductions.

src/qEmbyApp/components/collapsiblesection.h

@@ -4,6 +4,7 @@
 #include <QString>
 #include <QWidget>
 
+class QPropertyAnimation;
 class QPushButton;
 class QVBoxLayout;
 
@@ -22,6 +23,7 @@ private:
     QPushButton* m_toggle;
     QWidget* m_content;
     QVBoxLayout* m_contentLayout;
+    QPropertyAnimation* m_heightAnim = nullptr;
     bool m_expanded = false;
     QString m_title;
 };