Darken the dark theme, and hover a tree row like a menu

The client's own look-and-feel defaults now live beside LookAndFeelManager as
FlatLaf properties files, which FlatLaf loads on top of each theme's own,
instead of a handful of UIManager.put calls.

The dark theme is built on three surfaces keyed to the toolbar's #282828: the
window chrome, the tab strips and menu bar one step below it at #232323, and
the content the client shows — channel tree, chat log, text fields, lists — at
#1e1e1e. Selections are #364b6f, and channel names in the tree are white; the
chat lines about server events keep the blue they had, which channel names no
longer carry. Everything FlatLaf derives (borders, scroll bars, hover states)
follows from those.

A read-only editor pane is a chat log or an information view here, not a
disabled input, so it keeps the content background rather than FlatLaf's
inactive one; and since neither a text view nor a tree fills its viewport, the
space below the last line takes their background instead of the window's.

The row under the pointer is now washed in the same colour a hovered menu is,
rather than outlined. FlatLaf states that colour as a shift of the surface
beneath it, so it is resolved against the window background exactly as the
menu bar resolves it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ts3-client/swing/src/main/java/com/ts3client/ui/ServerTreePanel.java

@@ -161,7 +161,6 @@ public final class ServerTreePanel extends JScrollPane {
         tree.setPathEditable(nicknameEditor::editsPath);
         tree.setInvokesStopCellEditing(true);
         setViewportView(tree);
-        getViewport().setBackground(Theme.treeBg());
         // The icon strip is drawn against the viewport's right edge, so the blitted
         // pixels a scroll would reuse are stale; repaint the whole viewport instead.
         getViewport().setScrollMode(JViewport.SIMPLE_SCROLL_MODE);
@@ -279,6 +278,14 @@ public final class ServerTreePanel extends JScrollPane {
         ChannelMenu.build(channel, actions).show(tree, e.getX(), e.getY());
     }
 
+    @Override
+    public void updateUI() {
+        super.updateUI();
+        // The rows stop at the last channel, so the space below them belongs to the tree
+        // rather than to the window behind it.
+        if (getViewport() != null) getViewport().setBackground(Theme.treeBg());
+    }
+
     /** The path of the tree node showing {@code target}, or {@code null}. */
     private TreePath pathOf(Object target) {
         java.util.Enumeration<?> nodes = root.breadthFirstEnumeration();