Switch from the community-maintained @mcp-ui/client to the official Anthropic SDK for MCP app rendering.
Introduces useAppBridge hook that drives AppBridge + PostMessageTransport directly, giving us full
control over the spec protocol and proper propagation of _meta.ui.csp and _meta.ui.permissions from
tool definitions to the sandbox iframe.
Removes useMCPAppCallbacks and the AppRenderer dependency; all three render sites (ToolCall, UIResourceCarousel,
MCPUIResource) now use a plain iframe with useAppBridge. The existing mcp-sandbox.html proxy is unchanged.