fix: suspend hero animation when not visible

lib/mixins/tab_visibility_aware.dart

@@ -0,0 +1,8 @@
+/// Mixin for screens that need to react to tab visibility changes.
+///
+/// Used by MainScreen to pause expensive work (e.g. animation tickers)
+/// when the screen's tab is no longer visible, and resume it when shown again.
+mixin TabVisibilityAware {
+  void onTabShown();
+  void onTabHidden();
+}