Feature: UI Runtime + Vite plugin

[znicholasbrown]

We want to create a browser-safe runtime that injects tools/resources/etc into composable the UI components can use.

Additionally, a Vite plugin that pulls co-located UI components into dedicated build files that can be safely run in an iframe without losing references to server tools.

Tentative task list:

• Host context — iframe receives HostContext on init: theme (light/dark), ~50 standardised CSS custom properties, displayMode (inline/fullscreen/pip), containerDimensions, locale, timeZone, platform, deviceCapabilities, and safeAreaInsets; View can request a display mode change and receive resize notifications
• Bidirectional communication — View↔Host JSON-RPC 2.0 over postMessage; View calls tools/call, resources/read, sampling/createMessage, ui/message (inject a user turn into the conversation), ui/update-model-context, ui/open-link, ui/download-file, and ui/request-display-mode; app-provided tools let the View register ephemeral tools callable by the host or agent for the iframe's lifetime
• @fastmcp/ui-runtime — client-side package for use inside app iframes; useMCPApp() hook delivers typed toolInput, hostContext, callTool(), sendMessage(), updateModelContext(), and registerTool(); useTools<typeof server>() generic derives fully-typed call signatures (tool names, argument shapes, return types) from the server's exported type — eliminates stringly-typed tool calls and gives full IntelliSense inside the iframe; framework-agnostic core with React bindings included
• Vite plugin (@fastmcp/vite-plugin) — co-locate server handler and React component in a single .tool.tsx file; plugin extracts the default-exported component, bundles it for the browser, auto-generates the ui:// URI from the filename, registers the resource on the server, and wires _meta.ui.resourceUri — no URI strings written by hand; HMR for the UI component wired into fastmcp dev --reload