chore(playground): load docs examples from the registry catalog

[johnleider]

Summary

Playground loads docs examples from the official /registry/* catalog (same seed as vuetify add).

• Deep-links: ?example=dialog, ?example=dialog/basic, ?example=components/dialog/basic, optional ?registry=
• File → Open example… browser (Docs examples | Saved)
• Docs “Open in Playground” stays hash-based by default; short registry URLs with VITE_PLAYGROUND_REGISTRY=1
• CORS on /registry/* (already on master via docs: publish a static registry for vuetify add #721; branch rebased)

Depends on: #721 (merged — registry live at https://0.vuetifyjs.com/registry/index.json)

Test plan

• Rebase onto latest master (nginx conflict resolved — keep Allow-Headers)
• Prod registry reachable + CORS *
• Resolve dialog / basic item JSON + file list
• PR Checks green after push
• Spot-check File → Open example… in a local playground if needed

Related

• Seed: docs: publish a static registry for vuetify add #721
• CLI: feat(add): write v0 examples into a project from the registry cli#27
• Docs: docs(cli): document vuetify add registry and local library lifecycle #794