refactor(ui-core): UiCore becomes MyMesh's Listener for ui-new

ui-new hands MyMesh the UI Core (UITaskBase::meshListener()). The Core
applies the display filter, labels room posts, files history, keeps DM /
channel / room unread and the queue count, and reports incoming messages
and adverts as MessageArrived / AdvertHeard events. UITask now derives
UITaskBase + UiCoreHost: it answers "is this conversation on screen" and
keeps the selection after an insert, forwards not-yet-extracted parts
(room login / admin, bot device actions, removal cleanup, shutdown), and
turns events into alert / wake / sound, drained at the start and end of
its loop(). Mesh callbacks no longer touch the display or buzzer.

AbstractUITask is split into UITaskBase (lifecycle, link state, helpers)
and the Listener glue ui-orig / ui-tiny keep using unchanged. Screenshot
handler reaches ui_task directly instead of casting the Listener.

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

examples/companion_radio/main.cpp

@@ -326,7 +326,7 @@ void setup() {
   if (disp && the_mesh.getNodePrefs())
     disp->setBrightness(the_mesh.getNodePrefs()->display_brightness);
   ui_task.begin(disp, &sensors, the_mesh.getNodePrefs());  // still want to pass this in as dependency, as prefs might be moved
-  the_mesh.setListener(&ui_task);
+  the_mesh.setListener(ui_task.meshListener());
 #ifdef DISPLAY_HAS_BUSY_PUMP
   if (disp) disp->setBusyPumpFn(pumpRadioDuringDisplayBusyWait, nullptr);
 #endif