feat(desktop): hide native titlebar on Windows/Linux with overlay window controls

Use titleBarStyle: 'hidden' on all platforms. On Windows/Linux, enable
titleBarOverlay with dark theme colors for native min/max/close buttons.
Extend useTrafficLightPadding to return side ('left'|'right'|null) so
consumers apply padding on the correct side per platform.

packages/app/src/components/split-container.tsx

@@ -886,7 +886,10 @@ function SplitPaneView({
         style={[
           styles.paneTabs,
           isFocusModeEnabled &&
-            trafficLightPadding.left > 0 && { paddingLeft: trafficLightPadding.left },
+            trafficLightPadding.side && {
+              paddingLeft: trafficLightPadding.left,
+              paddingRight: trafficLightPadding.right,
+            },
         ]}
       >
         <WorkspaceDesktopTabsRow