Add Playwright E2E test for generate to editor to export

## Summary
No browser E2E tests exist. A happy-path test would catch regressions in the core user flow.

## Tasks
- [ ] Add Playwright to `apps/web`
- [ ] Test: start generation (mock AI) → land in editor → slides visible
- [ ] Test: export PPTX or PDF (poll job status)
- [ ] Run in CI with `AI_PROVIDER=mock`

## Files
- `apps/web/`
- `.github/workflows/ci.yml`
- `apps/api/src/services/ai/mock.ts`

Start by inspecting apps/web/ for the existing application and test setup, then read apps/api/src/services/ai/mock.ts and .github/workflows/ci.yml. Verify how the generation flow reaches the editor and how export job status is exposed before defining the Playwright steps. Done means the mock-backed generation, visible slides, and PPTX or PDF export run in CI with AI_PROVIDER=mock.

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