perf(ui): replace animated-sidebar drop shadows with pre-baked 9-patch overlay

QGraphicsDropShadowEffect forces the host widget through an off-screen
software Gaussian blur on every repaint. On the floating sidebar (HomeView,
blur 25) and the player right sidebar (PlayerView, blur 35), the blur was
re-rasterized on every frame of the 350ms slide animation, the main source
of UI jank.

Bake the blur once into a 9-patch PNG (cached to CacheLocation/shadows via
QGraphicsBlurEffect) and render it at runtime as a cheap 9-slice stretch with
zero per-frame blur cost:

- utils/shadowpixmap: off-screen bake + on-disk cache, dpr-aware.
- components/shadowoverlay: mouse-transparent sibling widget, lazy-baked,
  paints the 9-patch; optional body cutout for semi-transparent targets so
  the shadow does not bleed through the body.
- homeview/playerview: drop the effect; the overlay tracks the sidebar
  geometry via the existing eventFilter (Move/Resize/Show/Hide), so every
  setGeometry/move site and the slide animation drive it for free.

src/qEmbyApp/views/media/playerview.cpp

@@ -8,6 +8,7 @@
 #include "../../components/playerosdlayer.h"
 #include "../../components/playerstatisticsoverlay.h"
 #include "../../components/playersubtitlesettingsdialog.h"
+#include "../../components/shadowoverlay.h"
 #include "../../utils/mediaitemutils.h"
 #include "../../utils/mediasourcepreferenceutils.h"
 #include "../../utils/playerpreferenceutils.h"
@@ -26,7 +27,6 @@
 #include <QFileDialog>
 #include <QFileInfo>
 #include <QFontMetrics>
-#include <QGraphicsDropShadowEffect>
 #include <QHBoxLayout>
 #include <QListWidget>
 #include <QMainWindow>
@@ -1611,6 +1611,8 @@ void PlayerView::setupUi()
     updateDanmakuButtonState();
     updateMediaSwitcherButton();
     m_rightSidebar->raise();
+    if (m_rightSidebarShadow)
+        m_rightSidebarShadow->stackUnder(m_rightSidebar);
 }
 
 
@@ -1633,9 +1635,11 @@ void PlayerView::updateLoadingState()
         
         m_topHUD->raise();
         m_bottomHUD->raise();
-        m_logoLabel->raise();         
-        m_networkSpeedLabel->raise(); 
+        m_logoLabel->raise();
+        m_networkSpeedLabel->raise();
         m_rightSidebar->raise();
+        if (m_rightSidebarShadow)
+            m_rightSidebarShadow->stackUnder(m_rightSidebar);
         m_rightTrigger->raise();
         if (m_activePopup)
             m_activePopup->raise();
@@ -2224,11 +2228,15 @@ void PlayerView::setupRightSidebar()
     m_rightSidebar->setFixedWidth(220);
     m_rightSidebar->setGeometry(20000, 0, 220, 100);
 
-    auto *shadow = new QGraphicsDropShadowEffect(this);
-    shadow->setBlurRadius(35);
-    shadow->setColor(QColor(0, 0, 0, 150));
-    shadow->setOffset(-5, 0);
-    m_rightSidebar->setGraphicsEffect(shadow);
+    // Drop shadow via a pre-baked 9-patch overlay instead of
+    // QGraphicsDropShadowEffect, which would re-run a software Gaussian blur on
+    // every frame of the 350ms slide. Body cutout is on because the sidebar
+    // background is semi-transparent (the shadow must not bleed through it).
+    // The overlay tracks the sidebar's geometry in syncRightSidebarShadow().
+    m_rightSidebarShadow = new ShadowOverlay(this);
+    m_rightSidebarShadow->setShadow(35, 0, QColor(0, 0, 0, 150));
+    m_rightSidebarShadow->setBodyCutout(true);
+    m_rightSidebarShadow->hide();
 
     auto *layout = new QVBoxLayout(m_rightSidebar);
     layout->setContentsMargins(0, 15, 0, 15);
@@ -2374,6 +2382,26 @@ void PlayerView::hideRightSidebar(bool immediate)
     m_rightSidebarAnim->start();
 }
 
+void PlayerView::syncRightSidebarShadow()
+{
+    if (!m_rightSidebarShadow || !m_rightSidebar)
+    {
+        return;
+    }
+
+    if (!m_rightSidebar->isVisible())
+    {
+        m_rightSidebarShadow->hide();
+        return;
+    }
+
+    const int reach = m_rightSidebarShadow->reach();
+    m_rightSidebarShadow->setGeometry(
+        m_rightSidebar->geometry().adjusted(-reach, -reach, reach, reach));
+    m_rightSidebarShadow->stackUnder(m_rightSidebar);
+    m_rightSidebarShadow->show();
+}
+
 
 
 void PlayerView::stopAndReport()
@@ -2874,6 +2902,16 @@ bool PlayerView::eventFilter(QObject *watched, QEvent *event)
         return BaseView::eventFilter(watched, event);
     }
 
+    // Keep the drop-shadow overlay glued to the sidebar. The slide animation
+    // and layout both move/resize it, landing here as Move/Resize events, so the
+    // shadow follows every frame with no per-frame blur.
+    if (watched == m_rightSidebar &&
+        (event->type() == QEvent::Move || event->type() == QEvent::Resize ||
+         event->type() == QEvent::Show || event->type() == QEvent::Hide))
+    {
+        syncRightSidebarShadow();
+    }
+
     if (watched == m_mpvWidget || watched == this || watched == m_topHUD || watched == m_titleLabel ||
         watched == m_bottomHUD)
     {