refactor(companion): unify screen save-on-exit via UITask::savePrefsIfDirty()

The "_dirty bool, then if (_dirty) the_mesh.savePrefs() on exit" pattern was
duplicated across 9 screens with subtle divergence: some cleared the flag
after saving, some left it set and relied on onShow() to reset. Replace all
12 exit sites with a single _task->savePrefsIfDirty(flag) helper that saves
once only if dirty and always clears the flag, so the "did we touch flash?"
answer lives in one place and the reset is consistent.

Edit sites still mark the flag manually (inherent to change tracking); only
the persist-on-exit boilerplate is centralised.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.cpp

@@ -1758,6 +1758,13 @@ void UITask::setCurrScreen(UIScreen* c) {
   _next_refresh = 100;
 }
 
+bool UITask::savePrefsIfDirty(bool& dirty) {
+  if (!dirty) return false;
+  the_mesh.savePrefs();
+  dirty = false;
+  return true;
+}
+
 /*
   hardware-agnostic pre-shutdown activity should be done here
 */