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');
 			}
 		};
 

internal/api/middleware.go

@@ -54,6 +54,8 @@ func agentErrToHTTP(err error) (int, string, string) {
 		return http.StatusConflict, "conflict", err.Error()
 	case connect.CodePermissionDenied:
 		return http.StatusForbidden, "forbidden", err.Error()
+	case connect.CodeUnavailable:
+		return http.StatusServiceUnavailable, "no_hosts_available", "no servers available — try again later"
 	case connect.CodeUnimplemented:
 		return http.StatusNotImplemented, "agent_error", err.Error()
 	default:
@@ -108,6 +110,9 @@ func serviceErrToHTTP(err error) (int, string, string) {
 		return http.StatusForbidden, "forbidden", "forbidden"
 	case strings.Contains(msg, "invalid or expired"):
 		return http.StatusUnauthorized, "unauthorized", "invalid or expired credentials"
+	case strings.Contains(msg, "no online") && strings.Contains(msg, "hosts available"),
+		strings.Contains(msg, "no host has sufficient resources"):
+		return http.StatusServiceUnavailable, "no_hosts_available", "no servers available — try again later"
 	case strings.Contains(msg, "invalid"):
 		return http.StatusBadRequest, "invalid_request", "invalid request"
 	default: