Revert "fix(linux): make the frameless settings window edge readable"

This reverts commit bbb1be69a2.

src/features/settings/dialog/SettingsDialog.module.css

@@ -204,10 +204,9 @@
   color: var(--fg);
 }
 /* Linux draws no shadow for a frameless window (Wayland/KDE give undecorated
-   windows no CSD), so mark its edge in CSS instead. --border-window alone is
-   near-invisible on the dark theme, so blend the foreground tone into it. */
+   windows no CSD), so mark its edge in CSS instead. */
 .windowEdge {
-  box-shadow: inset 0 0 0 1px color-mix(in srgb, var(--fg) 32%, var(--border-window));
+  box-shadow: inset 0 0 0 1px var(--border-window);
 }
 .windowBody {
   flex: 1;