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>

frontend/src/components/Layout/MainLayout.tsx

@@ -42,7 +42,7 @@ export const MainLayout: React.FC = () => {
     <Layout style={{ height: '100vh', overflow: 'hidden' }}>
       <TopNav />
 
-      <Layout style={{ flex: 1, overflow: 'hidden' }}>
+      <div style={{ display: 'flex', flexDirection: 'row', flex: 1, overflow: 'hidden' }}>
         {/* Resizable sidebar */}
         <div
           style={{
@@ -89,7 +89,7 @@ export const MainLayout: React.FC = () => {
         >
           <SessionTabs />
         </Content>
-      </Layout>
+      </div>
     </Layout>
   );
 };