Leave all servers in parallel on shutdown

Quitting tells every server goodbye at once, so leaving takes as long as
the slowest server rather than all of them together. Desktop and Android
both go through ServerSession.shutdownAll.

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

ts3-client/core/src/main/java/com/ts3client/session/ServerSession.java

@@ -208,6 +208,27 @@ public final class ServerSession {
         if (isBusy()) conn.disconnectBlocking("Leaving");
     }
 
+    /**
+     * {@link #shutdown()}s every session at once, so leaving takes as long as the slowest
+     * server rather than all of them together: a frontend may have little time left.
+     */
+    public static void shutdownAll(List<ServerSession> sessions) {
+        List<Thread> leaving = new ArrayList<>();
+        for (ServerSession s : sessions) {
+            Thread t = new Thread(s::shutdown, "ts3-leave");
+            t.start();
+            leaving.add(t);
+        }
+        for (Thread t : leaving) {
+            try {
+                t.join();
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+                return;
+            }
+        }
+    }
+
     /** Releases the background resources of a session that is being thrown away. */
     public void dispose() {
         conn.getIcons().shutdown();

ts3-client/core/src/main/java/com/ts3client/session/Sessions.java

@@ -153,7 +153,7 @@ public final class Sessions {
 
     /** Leaves every server, waiting until each has been told. */
     public void shutdown() {
-        for (ServerSession s : new ArrayList<>(sessions)) s.shutdown();
+        ServerSession.shutdownAll(new ArrayList<>(sessions));
         handover.shutdown();
     }
 }