feat(split-screen): redesign split screen with free-form panes and pane picker

src/ui/tests/sidebar/AppRail.test.tsx

@@ -27,7 +27,6 @@ function renderRail(onRailClick = vi.fn()) {
     <AppRail
       railView="hosts"
       sidebarOpen={false}
-      splitMode="none"
       username="alice"
       isAdmin={false}
       onRailClick={onRailClick}

src/ui/tests/sidebar/rail-items.test.ts

@@ -60,7 +60,6 @@ describe("RAIL_ITEMS", () => {
       "quick-connect",
       "ssh-tools",
       "macros",
-      "split-screen",
       "sync",
     ]);
   });