add basic alert system to companion UI

Adds `_alert[80]` which can be set along with `_needs_refresh`
to trigger a simple on screen alert that lasts 1s at this time.

Implements POC with double press to advert action

examples/companion_radio/UITask.h

@@ -33,6 +33,7 @@ class UITask {
   char _version_info[32];
   char _origin[62];
   char _msg[80];
+  char _alert[80];
   int _msgcount;
   bool _need_refresh = true;
   bool _displayWasOn = false;  // Track display state before button press