Filter roles by department

Candidates should be able to narrow roles by department, and the filtering foundation should be reusable for additional facets like remote status or employment type.

Acceptance criteria

• The home page includes an accessible department filter.
• Filtering uses helpers in src/lib/ that operate on Job[] arrays.
• The filter can combine with remote or employment type filters if those are implemented.
• Empty states are clear when no roles match.
• Controls include stable data-testid attributes.
• Unit tests cover filtering behavior.
• Playwright tests verify department filtering.

Start at the home page and inspect the helpers in src/lib/ that operate on Job[] arrays. Run the existing unit-test and Playwright test commands to understand their conventions. Done means an accessible department filter with stable data-testid attributes, clear empty states, composable filtering, and coverage for the filtering behavior and department flow.