Add cohort-heatmap-server example

Demo MCP App visualizing cohort retention data with:
- Interactive heatmap with HSL color interpolation
- Multiple metrics: Retention %, Revenue Retention, Active Users
- Monthly/weekly period views
- Cell hover tooltips and row/column highlighting
- Light/dark theme support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

examples/cohort-heatmap-server/mcp-app.html

@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>Cohort Retention Heatmap</title>
+  <link rel="stylesheet" href="/src/global.css">
+</head>
+<body>
+  <div id="root"></div>
+  <script type="module" src="/src/mcp-app.tsx"></script>
+</body>
+</html>