add a persistent settings store

src/main/java/wtf/beatrice/autosqueal/util/SystemUtil.java

@@ -1,5 +1,6 @@
 package wtf.beatrice.autosqueal.util;
 
+import java.nio.file.Path;
 import java.util.Locale;
 
 public class SystemUtil
@@ -9,6 +10,27 @@ public class SystemUtil
         return fromOsName(System.getProperty("os.name"));
     }
 
+    /**
+     * The per-user directory where the app keeps its settings, following
+     * the conventions of the host platform.
+     */
+    public static Path getAppConfigDir() {
+        String home = System.getProperty("user.home", ".");
+        String appData = System.getenv("APPDATA");
+        String xdgConfigHome = System.getenv("XDG_CONFIG_HOME");
+
+        return switch (getHostSystem()) {
+            case MAC_OS -> Path.of(home, "Library", "Application Support", "autosqueal");
+            case WINDOWS -> appData != null && !appData.isBlank()
+                    ? Path.of(appData, "autosqueal")
+                    : Path.of(home, ".autosqueal");
+            case LINUX -> xdgConfigHome != null && !xdgConfigHome.isBlank()
+                    ? Path.of(xdgConfigHome, "autosqueal")
+                    : Path.of(home, ".config", "autosqueal");
+            default -> Path.of(home, ".autosqueal");
+        };
+    }
+
     public static OperatingSystem fromOsName(String osName) {
         String normalizedOsName = osName.toLowerCase(Locale.ENGLISH);
         if (normalizedOsName.contains("win")) {