Learn hub: match the live layout and drive articles from an index

[benpeter]

The /learn hub is visually different from the live continentaltire.com/learn, and its article cards are hardcoded. Two parts:

1. Index every /learn article into /learn/query-index.json so the hub lists them from the index.
2. Rebuild the hub toward the live layout: hero, category tab nav, category sections, an indexed article grid, product highlights, and the EV band.