Disable custom shadow painting for SeparatePanel on Wayland.

[gearonixx]

Problem

On Wayland, SeparatePanel renders its own client-side shadow via Shadow::paint() and applies st::callShadow.extend as padding. However, on Wayland the compositor already provides window shadows.

Root Cause

createBorderImage() and initGeometry() unconditionally applied st::callShadow.extend as shadow padding and drew the shadow, without checking whether the platform already provides compositor-level shadows.

Fix

Affected file

• ui/widgets/separate_panel.cpp (in desktop-app/lib_ui)

Testing

On Wayland SeparatePanel should display with compositor-provided shadow only

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