fix(app-shell): drop the dead initializer flagged by no-useless-assignment (#4509)

`let def = null` is never read — both the try and the catch either assign or
return — so ESLint's no-useless-assignment failed the Lint job. Declare without
the initializer. Also removes an unused React import from the new hook test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E5CYr5SDwe85gH2Jr5KSgu

Author: claude

packages/app-shell/src/hooks/__tests__/useNavActionDispatch.test.tsx

@@ -17,7 +17,6 @@
  */
 
 import { describe, it, expect, vi, beforeEach } from 'vitest';
-import React from 'react';
 import { renderHook, act } from '@testing-library/react';
 
 const execute = vi.fn();

packages/app-shell/src/hooks/useNavActionDispatch.ts

@@ -63,7 +63,7 @@ export function useNavActionDispatch(): (item: NavigationItem) => void {
         return;
       }
 
-      let def: ActionDefLike | null = null;
+      let def: ActionDefLike | null;
       try {
         def = (await getItem('action', actionName)) as ActionDefLike | null;
       } catch (err) {