Add Mago4 demo ERP with Python FastAPI

Implements a working in-memory ERP demo inspired by Mago4, covering
the core modules: dashboard with KPI widgets, customers, suppliers,
products, sales orders, purchase orders, and warehouse inventory.

- FastAPI backend with Pydantic models and REST API (auto-docs at /docs)
- Jinja2 HTML templates with Mago4-style UI (dark sidebar, blue/orange/
  green/red palette, Material Design icons)
- In-memory fake database seeded with sample customers, suppliers,
  products, and orders — ready to swap for SQLAlchemy + SQL Server
- Workarounds for Python 3.14 + Starlette 1.3 / Jinja2 compatibility
  (cache_size=0, new TemplateResponse(request, name, ctx) signature)
- launch.json for one-click preview; run.bat for manual startup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/launch.json

@@ -0,0 +1,12 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "mago4-demo",
+      "runtimeExecutable": "python",
+      "runtimeArgs": ["main.py"],
+      "port": 8000,
+      "autoPort": true
+    }
+  ]
+}