fix(app): float composer track pills consistently

Keep agent and task pills as a transparent transcript overlay while measuring their rendered height for scroll-tail clearance. Share geometry and spacing with the import-session pill so borders, type, corners, and composer separation match across platforms.

packages/app/src/agent-stream/turn-footer.test.tsx

@@ -6,7 +6,10 @@ import { createRoot, type Root } from "react-dom/client";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 vi.mock("react-native", () => ({
-  Platform: { OS: "web" },
+  Platform: {
+    OS: "web",
+    select: (options: { web?: unknown; default?: unknown }) => options.web ?? options.default,
+  },
   View: ({ children, testID }: { children?: React.ReactNode; testID?: string }) => (
     <div data-testid={testID}>{children}</div>
   ),