feat(ci): integrate Argos visual regression via vitest screenshots (#21210)

## Summary

- Adds `@argos-ci/storybook` vitest plugin to `twenty-ui` for automatic
screenshot capture during vitest storybook tests
- Uploads captured screenshots (PNG, ~5MB) as a CI artifact instead of
passing the full storybook build
- Updates the visual regression dispatch workflow to pass
`mode=argos-screenshots` to ci-privileged, which then uploads
screenshots to Argos via CLI

This replaces the 10-minute Storybook screenshot capture with a ~30s
vitest browser-mode approach. The heavy screenshot work happens on free
public runners, while ci-privileged only handles the Argos API upload
(keeping secrets private).

## Architecture

```
twenty (public, free runners)          ci-privileged (private)
─────────────────────────────          ────────────────────────
1. Build storybook-static              4. Download screenshots artifact
2. Vitest captures screenshots         5. `argos upload` → Argos API
3. Upload screenshots artifact         6. Poll for results
                                       7. Post PR comment
```

## Test plan

- [x] Verified locally: vitest captures 225 screenshots in ~28s
- [x] Verified `@argos-ci/cli upload` successfully creates Argos build
from captured screenshots
- [x] Argos diffs computed and results visible via API
- [ ] CI runs end-to-end on a PR

.gitignore

@@ -56,3 +56,4 @@ TRANSLATION_QA_REPORT.md
 .playwright-mcp/
 .playwright-cli/
 output/playwright/
+screenshots/