fix(ui): unify macOS shell top inset with sidebar/background surface (#702)

The 28px top inset that clears the macOS overlay traffic lights sat on the
content wrapper with no background, so it exposed the lighter WindowChrome
glass (--ol-window-bg) as a full-width band above the sidebar and panel —
reading as an extra top row that the Windows build (no inset) doesn't have.

Paint that wrapper with the same --ol-app-shell-bg the sidebar/background
already use, so the inset blends into one continuous surface with the panel
card floating on top. One property; token carries its own light + dark
values; no layout, token, or other UI changes.

Co-authored-by: sim <sim@local>

openless-all/app/src/components/FloatingShell.tsx

@@ -194,7 +194,7 @@ function FloatingShellBody({ os, initialTab, initialSettings }: { os: OS; initia
   const useOpaqueMain = mobile || os === 'linux';
 
   return (
-    <div style={{ flex: 1, position: 'relative', display: 'flex', flexDirection: 'column', minHeight: 0, paddingTop: mobile ? 0 : os === 'mac' ? 28 : 0 }}>
+    <div style={{ flex: 1, position: 'relative', display: 'flex', flexDirection: 'column', minHeight: 0, paddingTop: mobile ? 0 : os === 'mac' ? 28 : 0, background: 'var(--ol-app-shell-bg)' }}>
 
       {mobile && (
         <MobileTopBar