fix: use root-relative marketing links on previews

[emmajam]

Summary

• make marketing links use root-relative paths on non-production deployments
• keep the /developers mount for production deployments
• add coverage for preview vs production path generation

Verification

• VITE_USE_HTTP=true pnpm exec vitest run src/marketing/app/_lib/developersPaths.test.ts src/marketing/next-shims.test.ts src/components/DocsHeader.test.ts src/lib/docs-routing.test.ts
• VITE_USE_HTTP=true pnpm exec tsc --noEmit --pretty false --project tsconfig.json
• CI=true VITE_USE_HTTP=true pnpm run build

This PR intentionally does not include the Tempo Accounts destination change from docs#672.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
tempo-docs	Ready	Preview, Comment	Jul 13, 2026 6:32pm