Move the on_mouse_wheel handler before the cursor.is_over(bounds)
guard. The panel's nested compositor only routes axis events to the
applet surface when the pointer is over it, making the bounds check
redundant for scroll events. This fixes scroll-to-adjust-volume not
working on the audio applet after a fresh login, when the cursor
coordinates sent by the panel may not match the applet's surface-local
coordinate space.
