Make @repo/app build self-sufficient: build design-system first

The Render service's dashboard build command (pre-blueprint-sync) runs
plain `pnpm --filter @repo/app build`, which does not build workspace
dependencies — @repo/design-system dist/ is missing and next build dies
with Module not found. Building the dependency inside the app's build
script makes every invocation path work: turbo (^build, now redundant
but cheap), the render.yaml buildCommand, the stale dashboard command,
and Dockerfile.app.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>