chore: remove experimental frontends (web, android, playground, mobile)

Limpieza de los frontends de prueba (SPA React, app Kotlin, gateway playground,
binding gomobile) tras la fase de exploración. El bus (cmd/membershipd + pkg/*)
queda intacto y verde. Empezamos un frontend web nuevo desde cero, construido
de forma incremental. Todo lo borrado permanece en el historial git por si hay
que recuperar algo.

web/src/App.tsx

@@ -1,29 +0,0 @@
-import { useState } from "react";
-import { GatewayClient } from "./api";
-import type { Peer } from "./types";
-import { ConnectScreen } from "./components/ConnectScreen";
-import { ChatLayout } from "./components/ChatLayout";
-
-// Connection holds the live gateway client plus the identity it connected as.
-interface Connection {
-  client: GatewayClient;
-  peer: Peer;
-}
-
-// App is the root: it shows the connect screen until the user picks a gateway
-// URL and a peer name, then swaps to the full chat layout. Disconnecting drops
-// back to the connect screen.
-export function App() {
-  const [conn, setConn] = useState<Connection | null>(null);
-
-  if (!conn) {
-    return <ConnectScreen onConnect={(client, peer) => setConn({ client, peer })} />;
-  }
-  return (
-    <ChatLayout
-      client={conn.client}
-      peer={conn.peer}
-      onDisconnect={() => setConn(null)}
-    />
-  );
-}