feat: add ui/download-file method for host-mediated file downloads

Add a new ui/download-file JSON-RPC request that allows MCP App views to
request file downloads through the host. Since MCP Apps run in sandboxed
iframes where direct downloads are blocked (allow-downloads not set),
this provides a host-mediated mechanism for file exports.

Spec changes (draft):
- Add ui/download-file request to Requests (View → Host) section
- Add downloadFile capability to HostCapabilities
- Document host behavior requirements (confirmation dialog, filename
  sanitization, base64 decoding, size limits)

SDK changes:
- Add McpUiDownloadFileRequest/Result interfaces to spec.types.ts
- Add downloadFile() method to App class
- Add ondownloadfile setter to AppBridge class
- Add DOWNLOAD_FILE_METHOD constant
- Generate Zod schemas

Example:
- Add download demo to integration-server (exports JSON with server time)
- Capability check: only shows download button when host advertises
  downloadFile capability

Based on developer feedback requesting file export capabilities for
visualization tools, document editors, and data analysis apps.

examples/integration-server/src/mcp-app.tsx

@@ -137,6 +137,24 @@ function GetTimeAppInner({
     log.info("Open link request", isError ? "rejected" : "accepted");
   }, [app, linkUrl]);
 
+  const canDownload =
+    app.getHostCapabilities()?.downloadFile !== undefined;
+
+  const handleDownloadFile = useCallback(async () => {
+    const sampleContent = JSON.stringify(
+      { time: serverTime, exported: new Date().toISOString() },
+      null,
+      2,
+    );
+    log.info("Requesting file download...");
+    const { isError } = await app.downloadFile({
+      filename: "export.json",
+      content: sampleContent,
+      mimeType: "application/json",
+    });
+    log.info("Download", isError ? "rejected" : "accepted");
+  }, [app, serverTime]);
+
   return (
     <main
       className={styles.main}
@@ -182,6 +200,13 @@ function GetTimeAppInner({
         />
         <button onClick={handleOpenLink}>Open Link</button>
       </div>
+
+      {canDownload && (
+        <div className={styles.action}>
+          <p>Export current server time as JSON file</p>
+          <button onClick={handleDownloadFile}>Download File</button>
+        </div>
+      )}
     </main>
   );
 }