Add department pages

Candidates often browse by function. Add a prerendered page for each department that lists roles from that department using the existing role card.

Acceptance criteria

• A dynamic department route uses getStaticPaths() and prerender = true.
• Each department page lists only roles for that department and reuses the role card component.
• Department names on role cards or metadata link to the matching department page.
• A pure helper returns roles for a department from a Job[] array.
• Unit tests cover the department helper.
• Playwright tests verify a department link and department page.

Start with the existing role card, Job[] data, and current route structure to locate the department page entry point. Add the prerendered dynamic route, department helper, and links, then run the unit and Playwright tests; done means each page shows only its department's roles and links work from role cards or metadata.