fix: improve error feedback for terminal disconnects and host unavailability

Show "[session disconnected]" in terminal when PTY websocket closes cleanly.
Map scheduler and agent unavailability errors to 503 with user-friendly
message instead of leaking internal details.

frontend/src/lib/components/TerminalTab.svelte

@@ -332,6 +332,7 @@
 
 			if (s.state === 'connected') {
 				updateSession(id, { state: 'disconnected' });
+				int.term.write('\r\n\x1b[38;2;107;104;98m[session disconnected]\x1b[0m\r\n');
 			}
 		};