feat(dashboard): add real-time in-app stats with Chart.js

- dashboard_stats_bp.py: endpoints /pos/api/dashboard/stats and
  /pos/api/dashboard/stats/employees (sales today/month, hourly,
  top products, employee productivity)
- dashboard-stats.js: renders hourly sales bar chart and top products
  doughnut chart using Chart.js
- chart.umd.min.js: vendored Chart.js v4.4.2

pos/static/js/dashboard-stats.js

@@ -0,0 +1,99 @@
+/**
+ * dashboard-stats.js — In-app real-time charts using Chart.js
+ * Fetches /pos/api/dashboard/stats and renders hourly + top-products charts.
+ */
+(function () {
+  'use strict';
+
+  const token = localStorage.getItem('pos_token') || '';
+  if (!token) return;
+
+  function headers() {
+    return { 'Authorization': 'Bearer ' + token, 'Content-Type': 'application/json' };
+  }
+
+  function fmt(n) {
+    return '$' + parseFloat(n || 0).toLocaleString('es-MX', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
+  }
+
+  async function loadStats() {
+    try {
+      const res = await fetch('/pos/api/dashboard/stats', { headers: headers() });
+      if (!res.ok) return;
+      const data = await res.json();
+      renderHourlyChart(data.hourly_sales || []);
+      renderTopProductsChart(data.top_products || []);
+    } catch (e) {
+      console.error('[dashboard-stats] failed to load', e);
+    }
+  }
+
+  function renderHourlyChart(hourly) {
+    const ctx = document.getElementById('hourlySalesChart');
+    if (!ctx) return;
+    const labels = hourly.map(function (h) { return h.hour + ':00'; });
+    const totals = hourly.map(function (h) { return h.total; });
+
+    new Chart(ctx, {
+      type: 'bar',
+      data: {
+        labels: labels,
+        datasets: [{
+          label: 'Ventas ($)',
+          data: totals,
+          backgroundColor: 'rgba(245, 166, 35, 0.7)',
+          borderColor: 'rgba(245, 166, 35, 1)',
+          borderWidth: 1,
+          borderRadius: 4,
+        }]
+      },
+      options: {
+        responsive: true,
+        maintainAspectRatio: false,
+        plugins: { legend: { display: false } },
+        scales: {
+          x: { ticks: { color: '#888', font: { size: 10 } }, grid: { display: false } },
+          y: { ticks: { color: '#888', font: { size: 10 }, callback: function (v) { return '$' + (v / 1000).toFixed(0) + 'k'; } }, grid: { color: '#333' } }
+        }
+      }
+    });
+  }
+
+  function renderTopProductsChart(topProducts) {
+    const ctx = document.getElementById('topProductsChart');
+    if (!ctx) return;
+    const labels = topProducts.map(function (p) { return p.name.substring(0, 20); });
+    const revenues = topProducts.map(function (p) { return p.revenue; });
+
+    new Chart(ctx, {
+      type: 'doughnut',
+      data: {
+        labels: labels,
+        datasets: [{
+          data: revenues,
+          backgroundColor: [
+            '#F5A623', '#E85D75', '#4ECDC4', '#556270', '#C7F464',
+            '#FF6B6B', '#45B7D1', '#96CEB4', '#FFEAA7', '#DDA0DD'
+          ],
+          borderWidth: 0,
+        }]
+      },
+      options: {
+        responsive: true,
+        maintainAspectRatio: false,
+        plugins: {
+          legend: {
+            position: 'right',
+            labels: { color: '#ccc', font: { size: 10 }, boxWidth: 10 }
+          }
+        }
+      }
+    });
+  }
+
+  if (document.readyState === 'loading') {
+    document.addEventListener('DOMContentLoaded', loadStats);
+  } else {
+    loadStats();
+  }
+})();