docs: README architecture diagram shows stale folder name `editor-v2` and missing `ui` package

[kuishou68]

Bug

The Repository Architecture section in README.md has two inaccuracies:

1. Stale folder name – the code-block root label is editor-v2/ instead of editor/.
   Cloning the repo produces a directory called editor, not editor-v2. CONTRIBUTING.md and SETUP.md both use editor/ correctly.

2. Missing ui package – the packages list only shows core and viewer, but packages/ui (shared UI components) also exists and is documented in SETUP.md.

Current (incorrect)

editor-v2/
├── apps/
│   └── editor/          # Next.js application
├── packages/
│   ├── core/            # Schema definitions, state management, systems
│   └── viewer/          # 3D rendering components

Expected

editor/
├── apps/
│   └── editor/          # Next.js application
├── packages/
│   ├── core/            # Schema definitions, state management, systems
│   ├── viewer/          # 3D rendering components
│   └── ui/              # Shared UI components

Small documentation-only fix — happy to open a PR if helpful.

[kuishou68]

I've opened PR #245 to fix this issue.