Scale the interface fractionally on Linux

Java2D on X11 (and so under XWayland) truncates its scale to a whole
number, drawing 150 % at 100 %. On Linux, pin Java2D to 1 and hand the
full factor to FlatLaf, taken from XSETTINGS Xft/DPI, KWin's Xwayland
scale or GDK_SCALE, or chosen in Options → Design.

The client's own sizes now go through UIScale: borders, gaps, dialog and
row sizes, icons (rendered at the scaled size, so they stay crisp),
custom-painted components and font sizes in the HTML views.

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

ts3-client/core/src/main/java/com/ts3client/config/Settings.java

@@ -154,6 +154,8 @@ public final class Settings {
 
     /** Look-and-feel variant; see {@link Appearance}. */
     public Appearance appearance = Appearance.LIGHT;
+    /** Interface scale factor, 1.0 being 100 %; 0 follows the desktop. Read at startup. */
+    public double uiScale = 0;
 
     // ---- window chrome ----
     /** Whether the status bar at the bottom of the window is shown. */
@@ -263,6 +265,7 @@ public final class Settings {
         iconPack = props.getProperty("iconPack", iconPack);
         iconPackDir = props.getProperty("iconPackDir", iconPackDir);
         appearance = parseAppearance(props.getProperty("appearance"), appearance);
+        uiScale = parseD(props.getProperty("uiScale"), uiScale);
         showStatusBar = parseB(props.getProperty("showStatusBar"), showStatusBar);
         showMasterVolumeSlider = parseB(props.getProperty("showMasterVolumeSlider"), showMasterVolumeSlider);
         infoSplitRatio = parseD(props.getProperty("infoSplitRatio"), infoSplitRatio);
@@ -311,6 +314,7 @@ public final class Settings {
         props.setProperty("iconPack", iconPack);
         props.setProperty("iconPackDir", iconPackDir);
         props.setProperty("appearance", appearance.name());
+        props.setProperty("uiScale", Double.toString(uiScale));
         props.setProperty("showStatusBar", Boolean.toString(showStatusBar));
         props.setProperty("showMasterVolumeSlider", Boolean.toString(showMasterVolumeSlider));
         props.setProperty("infoSplitRatio", Double.toString(infoSplitRatio));