feat: enable limiting applications to a single instance

src/app/settings.rs

@@ -62,6 +62,9 @@ pub struct Settings {
 
     /// Whether the application should exit when there are no open windows
     pub(crate) exit_on_close: bool,
+
+    /// Only allow a single instance of the application to run
+    pub single_instance: bool,
 }
 
 impl Settings {
@@ -97,6 +100,7 @@ impl Default for Settings {
             theme: crate::theme::system_preference(),
             transparent: false,
             exit_on_close: true,
+            single_instance: false,
         }
     }
 }