MCP Apps ui:// widget never renders in Claude Desktop (Windows) — text-only via both stdio and remote

## Summary

A custom MCP server exposing an MCP App (ext-apps) tool with a `ui://` HTML resource shows only the tool's **text** result in Claude Desktop on **Windows** — the widget iframe is never mounted. Reproduces on **both** a local stdio server and a remote Streamable-HTTP connector.

## Environment

- Claude Desktop: latest, Windows 11
- `@modelcontextprotocol/sdk` 1.30.0, `@modelcontextprotocol/ext-apps` 1.7.5
- MCP protocol negotiated: `2025-11-25`

## Setup (matches the official ext-apps Quickstart exactly)

- `registerAppTool` with `_meta.ui.resourceUri` (the library normalizes to both `ui.resourceUri` and the flat `ui/resourceUri` key)
- `registerAppResource` returning `mimeType: text/html;profile=mcp-app`, a fully self-contained inline HTML page (no external assets / no CDN)
- Tool handler returns `{ content: [{ type: "text", text }] }`
- Widget: `new App({...}); app.ontoolresult = ...; await app.connect()`

## Verified

- `tools/list` shows the tool `_meta` carrying the `ui://` resourceUri; `resources/read` returns the HTML over the wire.
- The **exact same HTML resource renders correctly when opened directly in a browser** — so the widget itself works; only Claude's iframe mount is missing.

## Symptom

Claude shows the text result only; no iframe/widget is rendered. Identical behavior on local stdio and remote Streamable-HTTP.

## Question / request

Is MCP Apps UI rendering enabled in Claude Desktop on **Windows**? I cannot find any confirmed Windows-Desktop custom-server render (only macOS). Please enable / fix iframe rendering for custom MCP App resources on Windows Claude Desktop.

## Related

- modelcontextprotocol/ext-apps#671 (same symptom, Windows Desktop)