Move workspace gate background and root layout off useUnistyles

The workspace gate (loading/unreachable/missing) used surfaceWorkspace,
which clashed with the panels' surface0. Switch the gate shell to
surface0 so it matches the agent panel.

Also remove three hot-path useUnistyles() calls from _layout.tsx that
were re-rendering the whole app on every theme/breakpoint/insets change:
RootLayout and AppContainer move to StyleSheet.create, and the desktop
window-controls effect lifts into a small leaf component.

packages/app/src/screens/workspace/workspace-screen.tsx

@@ -1259,7 +1259,7 @@ function renderWorkspaceScreenGateShell(input: {
 
   return (
     <WorkspaceFocusProvider workspaceKey={input.workspaceKey}>
-      <View style={containerWithWorkspaceBackgroundStyle}>
+      <View style={styles.container}>
         <View style={styles.threePaneRow}>
           <View style={styles.centerColumn}>
             <WorkspaceScreenGateFrame>{input.gate}</WorkspaceScreenGateFrame>