feat(threejs): add visibility optimization, host styling, transparent background

- Add visibility-based animation pause (IntersectionObserver + wrapped rAF)
- Zero CPU usage when widget scrolls out of view
- Apply host style variables via useHostStyles
- Support transparent WebGL background (alpha: true)
- Use light-dark() for theme-aware fallback colors
- Tool no longer echoes code back in result
- Document performance features in README

examples/threejs-server/src/mcp-app-wrapper.tsx

@@ -5,7 +5,7 @@
  * props to the actual widget component.
  */
 import type { App, McpUiHostContext } from "@modelcontextprotocol/ext-apps";
-import { useApp } from "@modelcontextprotocol/ext-apps/react";
+import { useApp, useHostStyles } from "@modelcontextprotocol/ext-apps/react";
 import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js";
 import { StrictMode, useState, useCallback, useEffect } from "react";
 import { createRoot } from "react-dom/client";
@@ -78,6 +78,9 @@ function McpAppWrapper() {
     },
   });
 
+  // Apply host styling (theme, CSS variables, fonts)
+  useHostStyles(app);
+
   // Get initial host context after connection
   useEffect(() => {
     if (app) {