Fix marketing Vercel output config

[BunsDev]

Summary

• add a root vercel.json that builds the marketing app and points Vercel at apps/marketing/.next
• declare @okcode/marketing#build Turbo outputs as .next/** so Turbo no longer expects dist
• pin the marketing Next.js turbopack.root to the repo to avoid incorrect workspace-root detection

Verification

• bun fmt
• bun lint
• bun typecheck
• bun run build:marketing

Context

The Vercel build itself was succeeding, but deployment packaging failed afterward because the project was still configured to look for a dist output directory instead of the marketing app's Next.js output.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
okcode-marketing	Error		Apr 1, 2026 9:03am
v0-compute-the-platform-to-build	Ready	Preview, Open in v0	Apr 1, 2026 9:03am