Make sponsor header tappable in job listings

[JacobCoffee]

Summary

Each sponsor row on the Job Listings page now links to that sponsor's detail page — no dead headers.

• `.sponsor-header` becomes an `` with `routerLink` to `/app/tabs/sponsors/sponsor-detail/`, reusing the slug scheme from the Sponsors page
• Adds a right-side chevron to signal tappability, mirroring the role rows below it
• Threads a structured-postings code path through `processListings` / `parseRoles` / `filterListings` so listings that carry explicit `postings: [{title, location, url}]` render directly (with location shown) instead of going through markdown parsing; falls back to the existing HTML parser when no structured data is present

Test plan

• Tap a sponsor header in Job Listings → navigates to that sponsor's detail page
• Sponsor logo area also registers as a tap (single anchor wraps logo + name + level badge)
• Role rows underneath still open their external URL and don't trigger header navigation
• Sponsors whose API response includes `postings` render each posting as its own row with title + location
• Sponsors without structured postings still render via markdown parsing as before

[JacobCoffee]