Decouple Admin UI from Go Binary: Serve as Static Asset via static.fileserver

[intel352]

Decouple the admin UI (workflow/ui/) from the workflow server binary so it is built and served via static.fileserver just like any application UI.

Tasks

• Remove go:embed from module/ui_dist/ and module/api_workflow_ui.go.
• Build admin UI separately in CI; produce artifact for Docker and deployment.
• Allow admin/config.yaml (or flag/env) to specify root path for admin UI assets.
• Remove admin.WriteUIAssets and admin.InjectUIRoot.
• Ensure static.fileserver config is clear and matches admin config.
• Update deployment docs to describe standalone admin UI build and serve process.

Acceptance

• Admin UI can be deployed and updated independent of Go binary.
• Serving mechanism matches that of application UIs (including SPA fallback).

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Copilot Agent assigned (can be tackled in parallel).

Linked to broader UI refactor effort.