feat: serve the MCP server at /mcp on the local servers

Mount the local MCP server at /mcp on the development and preview
servers for clients that connect by URL. The route accepts only
requests from this machine that name a loopback host and, when an
Origin is sent, come from a loopback origin, and drains oversized
bodies so the 413 response is delivered. The provider-order regression
test now pins the route between the providers and the API fallback.

server/standalone/vite.config.js

@@ -2,6 +2,7 @@ import { fileURLToPath } from 'node:url';
 import { defineConfig, loadEnv } from 'vite';
 import { createBrowserViteConfig } from '../../build/vite.js';
 import { localProviderPlugins } from '../providers/local.js';
+import { localMcpPlugin } from '../mcp/plugin.js';
 import { apiNotFoundPlugin } from './api-not-found.js';
 
 const root = fileURLToPath(new URL('../../', import.meta.url));
@@ -13,7 +14,7 @@ export default defineConfig(({ command, mode }) => {
     if (process.env[key] === undefined) process.env[key] = value;
   }
   return createBrowserViteConfig({
-    plugins: [...localProviderPlugins(), apiNotFoundPlugin()],
+    plugins: [...localProviderPlugins(), localMcpPlugin(), apiNotFoundPlugin()],
     googleApiKey: process.env.GOOGLE_MAPS_API_KEY,
     cesiumToken: process.env.CESIUM_ION_TOKEN,
     host: process.env.HOST,