fix: mock authClient.useSession in App.test.tsx

Pre-existing test failure from Phase 1 better-auth migration.
Dashboard calls authClient.useSession() which makes an unresolved
async call in test environment. Mock it to return null session
(isPending: false) so the unauthenticated UI renders correctly.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/App.test.tsx

@@ -1,7 +1,13 @@
 import { render, screen } from '@testing-library/react'
-import { describe, it, expect } from 'vitest'
+import { describe, it, expect, vi } from 'vitest'
 import App from './App.tsx'
 
+vi.mock('./lib/auth-client.ts', () => ({
+  authClient: {
+    useSession: () => ({ data: null, isPending: false }),
+  },
+}))
+
 describe('App', () => {
   it('renders the dashboard on the root route', () => {
     render(<App />)