fix: stop versioning next-env.d.ts and generate it for type-check

[pettinarip]

Summary

next-env.d.ts has been showing up as a modified file on every next dev / next build since PR #18139 started tracking it. Two compounding causes:

1. Prettier round-trip: when ci: consolidate workflows into ci.yml #18139 committed the file, lint-staged ran Prettier on it. Our .prettierrc has semi: false, so the trailing semicolon Next.js writes on the import "./.next/types/routes.d.ts"; line got stripped. Next then sees a mismatch and rewrites it.
2. Dev vs build path divergence (Next 16): next dev uses .next/dev as distDir, next build uses .next. So the two modes write different paths (./.next/dev/types/routes.d.ts; vs ./.next/types/routes.d.ts;). No single committed content can satisfy both. This is tracked upstream in vercel/next.js#85738.

The official Next.js guidance is clear:

We recommend adding next-env.d.ts to your .gitignore file.
Running next dev, next build, or next typegen regenerates this file.

This PR follows that guidance:

• Re-adds next-env.d.ts to .gitignore (reverts the line removal in ci: consolidate workflows into ci.yml #18139).
• Untracks the file.
• Changes pnpm type-check from tsc --noEmit to next typegen && tsc --noEmit. next typegen ships in Next 15.5+ and generates next-env.d.ts + .next/types/{routes,validator,cache-life}.d.ts in ~600ms — no full build needed, and as a bonus the CI type-check now actually validates the typed-routes layer (it didn't before, because .next/types/ didn't exist in CI).

Test plan

• git status stays clean across pnpm dev and pnpm build locally
• CI type-check job passes
• First-time clone (no .next/) → pnpm type-check works end-to-end

References

• Next.js docs — TypeScript / next-env.d.ts
• vercel/next.js#85738 — next build and next dev generate different next-env.d.ts file
• vercel/next.js#47010 — How to generate next-env.d.ts in CI

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