[perf] Lazy-load ChallengeAuthoring on /challenge/create route

[github-actions]

Tier 3 — Loading Optimisation: Code Splitting via next/dynamic

Baseline

The /challenge/create route statically imported ChallengeAuthoring, bundling all 1 005 lines (across 5 files) into the route's initial JS chunk:

File	Lines
authoring-chat.tsx	334
quick-templates.tsx	204
index.tsx	146
authoring-message-list.tsx	141
authoring-input-form.tsx	66
Total	1 005

All of this code was downloaded and parsed before the user could interact with the page header or see any UI.

Fix

Replace the static import with next/dynamic({ ssr: false }):

// Before
import { ChallengeAuthoring } from '@/components/ChallengeAuthoring';

// After
const ChallengeAuthoring = dynamic(
  () => import('@/components/ChallengeAuthoring').then((mod) => mod.ChallengeAuthoring),
  { ssr: false, loading: () => (div className={styles.loading})(Spinner size="medium" /)(/div) }
);
```

- `ssr: false` — the page is already `'use client'`; no server-side rendering needed for this component.
- `loading` fallback — reuses the existing `.loading` CSS class (centred spinner), so the layout doesn't shift.
- The `AppHeader`, error `Banner`, and all hooks load instantly in the initial chunk; the authoring UI streams in as a separate chunk when the browser is ready.

Also replaced the one remaining `style=\{\{ flex: 1, overflow: 'hidden' }}` on the wrapper `(div)` with a new `.authoringWrapper` CSS class in `challenge.module.css`, keeping the file inline-style-free.

### Result

`ChallengeAuthoring` and its sub-components are now a separate JS chunk that loads after the critical path, reducing Time-to-Interactive for `/challenge/create`.

### Test update

`create-page.test.ts` was updated to use `await act(async () => ...)` in `renderPage()`. This is required because `next/dynamic` wraps the import in `React.lazy`, which resolves asynchronously even when the module is mocked synchronously by Vitest. Awaiting `act` flushes the microtask queue so the lazy boundary resolves before assertions run. All 3 tests in the file continue to pass.

### Verification

```
npx tsc --noEmit   → clean (0 errors)
npm run lint       → clean (0 warnings)
npm test           → 1 025 passed (3 pre-existing GitHub-auth failures unrelated to this change)

Generated by Daily Performance Improver

[github-actions]

Pull request created: #102