Add basic-server-vanillajs example

Demonstrates MCP App SDK usage with vanilla JavaScript (no framework),
providing parity with the existing basic-server-react example.

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

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

examples/basic-server-vanillajs/README.md

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+# Example: Basic Server (Vanilla JS)
+
+An MCP App example with a vanilla JavaScript UI (no framework).
+
+> [!TIP]
+> Looking for a React-based example? See [`basic-server-react`](https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/basic-server-react)!
+
+## Overview
+
+- Tool registration with a linked UI resource
+- Vanilla JS UI using the [`App`](https://modelcontextprotocol.github.io/ext-apps/api/classes/app.App.html) class directly
+- App communication APIs: [`callServerTool`](https://modelcontextprotocol.github.io/ext-apps/api/classes/app.App.html#callservertool), [`sendMessage`](https://modelcontextprotocol.github.io/ext-apps/api/classes/app.App.html#sendmessage), [`sendLog`](https://modelcontextprotocol.github.io/ext-apps/api/classes/app.App.html#sendlog), [`sendOpenLink`](https://modelcontextprotocol.github.io/ext-apps/api/classes/app.App.html#sendopenlink)
+
+## Key Files
+
+- [`server.ts`](server.ts) - MCP server with tool and resource registration
+- [`mcp-app.html`](mcp-app.html) / [`src/mcp-app.ts`](src/mcp-app.ts) - Vanilla JS UI using `App` class
+
+## Getting Started
+
+```bash
+npm install
+npm run dev
+```
+
+## How It Works
+
+1. The server registers a `get-time` tool with metadata linking it to a UI HTML resource (`ui://get-time/mcp-app.html`).
+2. When the tool is invoked, the Host renders the UI from the resource.
+3. The UI uses the MCP App SDK API to communicate with the host and call server tools.
+
+## Vanilla JS Pattern
+
+```typescript
+import { App, PostMessageTransport } from "@modelcontextprotocol/ext-apps";
+
+// Get element references from static HTML
+const button = document.getElementById("my-button")!;
+
+// Create app instance
+const app = new App({ name: "My App", version: "1.0.0" });
+
+// Register handlers BEFORE connecting
+app.ontoolresult = (result) => {
+  /* handle result */
+};
+app.onerror = console.error;
+
+// Add event listeners
+button.addEventListener("click", () => {
+  /* ... */
+});
+
+// Connect to host
+app.connect(new PostMessageTransport(window.parent));
+```