feat(web): add Phoenix marketing site

Summary

• Add a Phoenix 1.8 app under web/ for the Once marketing site.
• Create a marketing home page with plain CSS and data-part selectors aligned with Atlas conventions.
• Add a /registry preview page backed by static Once rule and script seed data.
• Pin Erlang and Elixir in mise.toml so the web app runs through the repository toolchain.

Testing

• mise exec -- mix precommit
• mise exec -- mix assets.build
• curl -fsS http://localhost:4000/ | rg "Once turns scripts|Browse the registry"
• curl -fsS http://localhost:4000/registry | rg "Once rules and scripts|release/package"

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