Paint the client with FlatLaf, in a light and a dark theme

The look-and-feel is FlatLaf now, chosen light or dark on the new Design
tab and remembered in the settings. Theme keeps only the colours TeamSpeak
gives meaning to (talking, away, muted, channel and server names) and takes
its surfaces and fonts from the look-and-feel, so a switch recolours the
whole window, chat logs included: the HTML panes share one themed stylesheet
and re-parse their content when the theme changes.

Also fixes the channel tree not scrolling: the tree listened for the mouse
wheel to re-check its hover row, and AWT only forwards a wheel event to the
enclosing scroll pane when the component under the pointer has no wheel
listener of its own. The hover row now follows the viewport instead, which
also covers scrollbar drags and keyboard scrolling.

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

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

@@ -32,7 +32,7 @@ public final class SettingsDialog extends JDialog {
     private final DevicesPanel devicesPanel;
     private final VoiceActivationPanel voiceActivationPanel;
     private final NotificationsPanel notificationsPanel;
-    private final IconPackPanel iconPackPanel;
+    private final DesignPanel designPanel;
     private final HotkeysPanel hotkeysPanel;
     private final ClientVersionPanel clientVersionPanel;
 
@@ -46,7 +46,7 @@ public final class SettingsDialog extends JDialog {
         this.onApply = onApply;
 
         notificationsPanel = new NotificationsPanel(settings, sounds);
-        iconPackPanel = new IconPackPanel(settings);
+        designPanel = new DesignPanel(settings);
         hotkeysPanel = new HotkeysPanel(hotkeys);
         clientVersionPanel = new ClientVersionPanel(settings);
         devicesPanel = new DevicesPanel(settings, livePlayback,
@@ -58,7 +58,7 @@ public final class SettingsDialog extends JDialog {
         tabs.addTab("Playback / Capture", scrollable(devicesPanel));
         tabs.addTab("Voice Activation", scrollable(voiceActivationPanel));
         tabs.addTab("Notifications", notificationsPanel);
-        tabs.addTab("Design", iconPackPanel);
+        tabs.addTab("Design", designPanel);
         tabs.addTab("Hotkeys", hotkeysPanel);
         tabs.addTab("Client Version", scrollable(clientVersionPanel));
 
@@ -128,7 +128,7 @@ public final class SettingsDialog extends JDialog {
     private void apply() {
         writeAudioSettings(settings);
         notificationsPanel.apply();
-        iconPackPanel.apply();
+        designPanel.apply();
         clientVersionPanel.apply();
         hotkeysPanel.apply();
         settings.save();
@@ -156,7 +156,7 @@ public final class SettingsDialog extends JDialog {
     /** Leaves without applying, putting back the settings that preview themselves live. */
     private void cancel() {
         notificationsPanel.revert();
-        iconPackPanel.revert();
+        designPanel.revert();
         close();
     }