feat: add rate-limited update check with desktop notification

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/ahfail-gtklock/src/handler.rs

@@ -86,6 +86,10 @@ impl WindowHandler {
                     data.ready_players.push(new_player);
                 }
             });
+
+            std::thread::spawn(|| {
+                ahfail_ui::update::check_for_update(ahfail_ui::VERSION);
+            });
         });
 
         unsafe {