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>

templates/customers.html

@@ -0,0 +1,136 @@
+{% extends "base.html" %}
+
+{% block title %}Clienti - Mago4 Demo{% endblock %}
+{% block page_title %}Clienti{% endblock %}
+
+{% block content %}
+<div class="page-header">
+    <button class="btn btn-primary" onclick="openCreateCustomerModal()">
+        <i class="material-icons">add</i> Nuovo Cliente
+    </button>
+</div>
+
+<div class="widget-card">
+    <div class="widget-header">
+        <h3 class="widget-title">Elenco Clienti</h3>
+    </div>
+    <div class="widget-body">
+        <table class="data-table">
+            <thead>
+                <tr>
+                    <th>Codice</th>
+                    <th>Ragione Sociale</th>
+                    <th>Città</th>
+                    <th>Paese</th>
+                    <th>Telefono</th>
+                    <th>Limite Credito</th>
+                    <th>Saldo</th>
+                    <th>Azioni</th>
+                </tr>
+            </thead>
+            <tbody>
+                {% for customer in customers %}
+                <tr>
+                    <td><strong>{{ customer.code }}</strong></td>
+                    <td>{{ customer.name }}</td>
+                    <td>{{ customer.city }}</td>
+                    <td>{{ customer.country }}</td>
+                    <td>{{ customer.phone }}</td>
+                    <td class="text-right">€ {{ "%.2f"|format(customer.credit_limit) }}</td>
+                    <td class="text-right {% if customer.balance > 0 %}text-danger{% endif %}">
+                        € {{ "%.2f"|format(customer.balance) }}
+                    </td>
+                    <td>
+                        <button class="btn-small btn-info" onclick="viewCustomer({{ customer.id }})">
+                            <i class="material-icons">visibility</i>
+                        </button>
+                        <button class="btn-small btn-warning" onclick="editCustomer({{ customer.id }})">
+                            <i class="material-icons">edit</i>
+                        </button>
+                    </td>
+                </tr>
+                {% endfor %}
+            </tbody>
+        </table>
+    </div>
+</div>
+
+<!-- Modal Detail -->
+<div id="customerModal" class="modal" onclick="closeModal(event)">
+    <div class="modal-content" onclick="event.stopPropagation()">
+        <div class="modal-header">
+            <h2 id="modalTitle">Dettagli Cliente</h2>
+            <button class="btn-close" onclick="closeModal()">
+                <i class="material-icons">close</i>
+            </button>
+        </div>
+        <div class="modal-body" id="modalBody">
+            <!-- Content loaded via JS -->
+        </div>
+    </div>
+</div>
+
+<script>
+function viewCustomer(customerId) {
+    fetch(`/api/customers/${customerId}`)
+        .then(r => r.json())
+        .then(data => {
+            document.getElementById('modalTitle').textContent = `Cliente: ${data.name}`;
+            document.getElementById('modalBody').innerHTML = `
+                <div class="detail-grid">
+                    <div class="detail-item">
+                        <label>Codice</label>
+                        <span>${data.code}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Ragione Sociale</label>
+                        <span>${data.name}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Email</label>
+                        <span>${data.email}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Telefono</label>
+                        <span>${data.phone}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Indirizzo</label>
+                        <span>${data.address}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Città</label>
+                        <span>${data.city}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Paese</label>
+                        <span>${data.country}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Limite Credito</label>
+                        <span>€ ${data.credit_limit.toFixed(2)}</span>
+                    </div>
+                    <div class="detail-item">
+                        <label>Saldo</label>
+                        <span class="${data.balance > 0 ? 'text-danger' : ''}">${data.balance > 0 ? '€ ' : ''}${Math.abs(data.balance).toFixed(2)}</span>
+                    </div>
+                </div>
+            `;
+            document.getElementById('customerModal').classList.add('show');
+        });
+}
+
+function editCustomer(customerId) {
+    alert('Modifica cliente: ' + customerId + ' (da implementare)');
+}
+
+function openCreateCustomerModal() {
+    alert('Creazione nuovo cliente (da implementare)');
+}
+
+function closeModal(event) {
+    if (event && event.target.id !== 'customerModal') return;
+    document.getElementById('customerModal').classList.remove('show');
+}
+</script>
+{% endblock %}