refactor: port upstream MyMesh::Listener; MyMesh no longer calls the UI directly

Ports upstream's "Abstract UI overhaul" (PR #3431 + follow-ups) with
identical names/signatures. MyMesh talks to the UI only through
MyMesh::Listener (setListener()); a second block holds Solo extensions
(own-send mirroring, relay echoes, room login/admin replies, [LOC] shares,
contact/channel removal, bot device actions, requestShutdown), all no-op
by default.

AbstractUITask becomes the Listener and carries the glue MyMesh used to
run for the UI (display filter, room-post labels, notifications,
connection polling). BLE PIN randomisation moves to main.cpp as upstream.
Also fixes ui-orig's stale advertReceived event on buzzer boards.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-tiny/UITask.cpp

@@ -581,6 +581,7 @@ bool UITask::isButtonPressed() const {
 }
 
 void UITask::loop() {
+  pollConnection();   // BLE link state -> hasConnection() (see AbstractUITask)
   char c = 0;
 #if UI_HAS_JOYSTICK
   int ev = user_btn.check();