Fix layout bug, tabs height, and RDP tunnel UUID handshake

- MainLayout: replace inner <Layout> with row-flex div so sidebar and
  session tabs appear side-by-side instead of stacked vertically
- global.css: add Ant Design Tabs CSS overrides so tab pane content
  fills available height (SSH terminal and RDP canvas sized correctly)
- rdp.ts: send guacd's ready-UUID as first WebSocket message so
  Guacamole.WebSocketTunnel completes its tunnel handshake correctly
- RdpTab: add connecting/error/disconnected status overlays for
  visibility when RDP fails

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/websocket/rdp.ts

@@ -1,4 +1,5 @@
 import { createConnection, Socket } from 'net';
+import { randomUUID } from 'crypto';
 import { FastifyInstance, FastifyRequest } from 'fastify';
 import { SocketStream } from '@fastify/websocket';
 import { WebSocket } from 'ws';
@@ -165,7 +166,12 @@ export async function rdpWebsocket(fastify: FastifyInstance) {
           throw new Error(`guacd handshake failed: expected 'ready', got '${readyInstruction[0]}'`);
         }
 
-        // 5. Flush any buffered bytes that arrived after 'ready'
+        // 5. Send the guacd connection UUID as the first WebSocket message.
+        //    Guacamole.WebSocketTunnel expects this as its tunnel-UUID handshake.
+        const guacdUUID = readyInstruction[1] ?? randomUUID();
+        socket.send(guacdUUID);
+
+        // 6. Flush any buffered bytes that arrived after 'ready'
         if (tcpBuf.value.length > 0 && socket.readyState === WebSocket.OPEN) {
           socket.send(tcpBuf.value);
           tcpBuf.value = '';