Load InterDisplay on production (headings silently fall back to Inter)

[joshestein]

🧩 Summary

interDisplay (next/font, src/lib/fonts.ts) is imported only in _document.tsx. The pages router emits next/font CSS only for fonts reachable from _app, and _app.tsx imports only inter — so --font-inter-display resolves empty in prod builds. globals.css points .bluedot-h1 / .bluedot-h2 at that empty var, so every website heading renders Inter on prod/staging while local dev renders true InterDisplay.

Other monorepo apps are unaffected: the library's .bluedot-h1 uses font-family: InterDisplay from the bluedot.org/fonts @font-face.

Found during #2790.

✅ Definition of done

1. interDisplay reachable from _app so its CSS ships
2. Verified on staging (not local dev) that .bluedot-h1 / .bluedot-h2 compute to InterDisplay
3. Design sign-off — this flips every heading on prod, so it's a sitewide visual change
4. Homepage hero stays pinned to Inter (deliberate inline style override)

📌 Notes / References

Blocks closing #2735 (parent DoD: "typeface across all apps"). Don't compare dev vs prod typography until this lands.