Add color-scheme meta tag to all example apps

Prevents iframe from automatically applying a default background color.

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

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

examples/basic-server-react/mcp-app.html

@@ -3,6 +3,7 @@
 <head>
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <meta name="color-scheme" content="light dark">
   <title>Get Time App</title>
   <link rel="stylesheet" href="/src/global.css">
 </head>