Add a careers summary

The home page should quickly communicate the scale of current hiring. Add a summary that shows total open roles and the number of departments hiring.

Acceptance criteria

• The home page displays the total number of open roles.
• The home page displays the number of departments with open roles.
• Empty states are graceful if there are no roles.
• Summary values come from a pure helper over Job[].
• Unit tests cover normal and empty summary states.
• Playwright tests verify the summary renders.

Start at the home page entry point and locate the existing Job[] data shape and the helper area. Add the careers summary with graceful empty-state handling, then run the unit tests and Playwright tests. Done means both normal and empty summary states are covered and the home page displays open-role and hiring-department totals.