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/InfoPanel.java

@@ -55,6 +55,7 @@ public final class InfoPanel extends JScrollPane {
             }
         });
         setViewportView(pane);
+        HtmlStyles.fillViewport(pane);
         clear();
     }