refactor: dedupe split-view render, strengthen persistence test

- Extract ConsoleArea helper to eliminate ~60 lines of duplicated layout
  toggle + grid rendering shared between currentLane and !currentLane branches
- Fix persistence test to validate the component's actual write path instead
  of manually re-seeding localStorage (proves writeSplitViewState is called)
- Remove orphaned grid/pane code left by incomplete merge

client/src/pages/__tests__/Workspace.test.tsx

@@ -671,12 +671,6 @@ describe("split terminal view", () => {
     await settle();
     unmount();
 
-    localStorage.clear();
-    localStorage.setItem(
-      "ccam.workspace.splitView",
-      JSON.stringify({ layout: 2, paneLaneIds: [null, lanesToReturn[1]!.id] })
-    );
-
     await renderWorkspace();
     const persistedSelects = screen.getAllByTestId("pane-lane-select");
     expect(persistedSelects).toHaveLength(2);