feat(gateway): add web UI with dashboard and chat interface

Refactor GatewayServer to serve HTTP and WebSocket on a shared
http.Server. Add static file serving with path traversal protection,
a dark-themed dashboard (system health, sessions, tools) and a
WebSocket chat interface with streaming tool events and markdown
rendering.

src/gateway/index.ts

@@ -6,6 +6,7 @@ export { SessionBridge } from './session-bridge.js';
 export type { SessionBridgeConfig } from './session-bridge.js';
 export { authenticateRequest } from './auth.js';
 export type { AuthConfig, AuthResult } from './auth.js';
+export { serveStatic } from './static.js';
 export {
   ErrorCode,
   isValidRequest,