feat: F4.6g1 the echo server and the generic client windows

Oracle EchoWindowRun uses Java's echo server and client windows as a
person does; SaveAfterRunRun echo saves what Java keeps after the run.
The room takes setActive with a port, connect/disconnect/send, sends
both views and writes the server's port and the client's target back.
The editor's windows show Java's buttons, fields and lines; a port that
is no port is marked and not handed over (Java's button throws).
Core fix: an empty message (one empty segment with the push, as Java's
erstelleSegmente makes) counted as a repeat and never arrived.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/editor/src/desktop/desktop.css

@@ -1026,3 +1026,60 @@
 .image-viewer-picture img {
   max-width: 100%;
 }
+
+/* The echo server and the generic client (GUIApplicationServerBausteinWindow,
+ * GUIApplicationClientBausteinWindow), on the web server's look. */
+.echo-server input,
+.simple-client input,
+.simple-client textarea {
+  min-height: 32px;
+  padding: 0 6px;
+  border: 1px solid #a0a0a0;
+  background: #ffffff;
+  color: #000000;
+  font: 16px Dialog, sans-serif;
+}
+
+.echo-server input {
+  width: 8em;
+}
+
+.echo-server [aria-invalid='true'],
+.simple-client [aria-invalid='true'] {
+  border-color: #ff0000;
+  color: #ff0000;
+}
+
+.echo-server input:read-only,
+.simple-client input:read-only {
+  background: #eeeeee;
+}
+
+.simple-client-field,
+.simple-client-message {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  flex: none;
+}
+
+.simple-client-field span {
+  flex: none;
+  width: 140px;
+}
+
+.simple-client-field input {
+  flex: 1;
+  min-width: 0;
+}
+
+.simple-client-message {
+  flex-direction: column;
+  align-items: stretch;
+}
+
+.simple-client-message textarea {
+  min-height: 3.5em;
+  padding: 4px 6px;
+  resize: vertical;
+}