fix(cultures): hide action toolbar when no cultures exist

[stipsitzm]

Motivation

• When a project contains zero cultures the page showed the filter/search empty-state and an irrelevant action toolbar with mostly-disabled buttons, which confuses the onboarding experience.

Description

• Render the action toolbar only when the project contains cultures by guarding it with cultures.length > 0 in frontend/src/pages/Cultures.tsx.
• Keep the existing CultureDetail empty-state logic where cultures.length === 0 shows the onboarding empty-state and cultures.length > 0 && filteredCultures.length === 0 shows the filter empty-state.
• No UI text or i18n keys were changed.

Testing

• Ran unit tests with cd frontend && npm run test -- src/__tests__/CultureDetail.test.tsx src/__tests__/CulturesActionArea.test.tsx and all tests passed.
• Ran linter with cd frontend && npm run lint and it reported pre-existing, repo-wide ESLint issues unrelated to this change.

---

Codex Task