Initial commit: step2urdf tool with handtuned JSON import.

Includes FastAPI backend, vendored step2urdf frontend, and A7 handtuned arm JSON for URDF generation.

frontend/UPSTREAM.md

@@ -0,0 +1,28 @@
+# Frontend origin (step2urdf)
+
+Primary UI is vendored from the open-source **step2urdf** project (same product as https://step2urdf.top/).
+
+| Field | Value |
+|-------|--------|
+| Upstream | https://github.com/Democratizing-Dexterous/step2urdf |
+| License | MIT |
+| Cloned commit | `5c67a6768ce6767edf31aaa9e0737561363c315e` (`5c67a67 Update README.md`) |
+| Local path | `frontend/` |
+
+## Local extensions
+
+- Vite `/api` proxy → FastAPI `127.0.0.1:8787` (Zhipu propose; API key never in frontend)
+- `ZhipuAssistDialog` + `applyZhipuDraft` wired into `URDFLeftPanel` (“智谱建议关节”)
+- `package.json` → `pnpm.onlyBuiltDependencies` for esbuild postinstall
+- `.npmrc` → `ignore-dep-scripts=false` so WASM/tooling install scripts run
+
+To refresh from upstream (this directory may still contain a nested `.git` from the clone):
+
+```bash
+cd frontend
+git fetch origin
+git log -1 --oneline
+# re-apply local patches if needed (Zhipu panel, vite proxy, UPSTREAM.md)
+```
+
+If you prefer a monorepo without nested git, remove `frontend/.git` manually and keep this file as the provenance record.