Work tree rows by their whole line, and outline the hovered one

Swing hit-tests a tree row only as far as its label, so a click past the
end of the name did nothing. Rows are now found by their vertical band
alone, which covers selection, the context menus, double-click join and
middle-click info; the expand handle to the left of a label still works
the row without selecting it.

The hovered row is outlined across that same full width, so what a click
would act on is visible before making it. Spacers, which nothing can be
done with, are skipped, and the outline gives way to the drag-and-drop
indicator while a drag is in progress.

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

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

@@ -9,6 +9,8 @@ public final class Theme {
     public static final Color WINDOW_BG = new Color(0xF0F0F0);
     public static final Color TREE_BG = new Color(0xFFFFFF);
     public static final Color TREE_SELECTION = new Color(0xCFE3FB);
+    /** Outline drawn around the row the pointer is over. */
+    public static final Color TREE_HOVER = new Color(0x9CC4EE);
     public static final Color TREE_TEXT = new Color(0x1E1E1E);
     public static final Color CHANNEL_TEXT = new Color(0x21486B);
     public static final Color SERVER_TEXT = new Color(0x123456);