Structure a monorepo running multiple Next.js apps without duplicating your design system

<context>
Apps that need to live in this monorepo: marketing site (apps/web), customer dashboard (apps/dashboard), internal admin tool (apps/admin)
What should be shared across them (UI components, types, config, utilities): A design-system UI package, shared TypeScript types for the API, shared eslint/tsconfig
Package manager: pnpm
</context>

<task>
Design the monorepo layout for these apps using Turborepo and pnpm workspaces, with shared code living in internal packages that each app depends on via the workspace protocol — not copy-pasted between app directories.
</task>

<structure_requirements>
- apps/ holds one folder per deployable Next.js app, each with its own next.config, package.json, and env files — Next.js only loads .env files from the app's own root directory, not the repo root, so each app manages its own environment variables even for values that happen to be the same across apps.
- packages/ holds shared, non-deployable code: a UI package for shared components, a config package for shared eslint/tsconfig/tailwind config, a types package if there's a shared domain model, each with its own package.json.
- Apps depend on internal packages via the workspace protocol (e.g. "@repo/ui": "workspace:*"), so changes to a shared package are picked up by every app that depends on it without a publish step.
- Configure turbo.json's task pipeline so that build/lint/test tasks declare their real dependencies (a package's build must complete before an app that depends on it builds), and so turbo's caching can skip rebuilding an app when nothing it actually depends on changed — don't leave every task marked as depending on the whole repo.
- Decide, and state explicitly, whether each app deploys independently (separate Vercel projects/domains) or as part of one deployment — this affects whether env vars and preview deployments are configured per-app or centrally.
</structure_requirements>

<output_format>
The folder tree (apps/ and packages/ with their contents), an example turbo.json pipeline entry showing the dependency between a shared package's build and an app's build, and one paragraph on the env-file gotcha above.
</output_format>