Fix workspace navigation regression on web (#772)

Centralize workspace navigation behind a small route-derived API so the workspace shell reads active selection from the current Expo Router route instead of a separately synchronized store.

Remove the stale layout-driven active-workspace write that regressed web after 02e5b564.

packages/app/src/components/worktree-setup-callout-source.test.tsx

@@ -67,7 +67,7 @@ vi.mock("expo-router", () => ({
 }));
 
 vi.mock("@/stores/navigation-active-workspace-store", () => ({
-  useNavigationActiveWorkspaceSelection: () => activeSelection.value,
+  useActiveWorkspaceSelection: () => activeSelection.value,
 }));
 
 vi.mock("@/stores/session-store-hooks", () => ({