fix lock surface background opacity to prevent compositor fallback bleedthrough

shell/lock/LockSurface.qml

@@ -15,6 +15,14 @@ WlSessionLockSurface {
 
     property real _unlockFade: 1
 
+    // Solid background - WlSessionLockSurface.color can be unreliable with
+    // Wayland compositors, so paint an explicit opaque rect to prevent the
+    // compositor fallback color (niri red) from bleeding through.
+    Rectangle {
+        anchors.fill: parent
+        color: S.Theme.base00
+    }
+
     // Clear desktop screenshot from ScreenshotService - visible immediately
     Image {
         anchors.fill: parent