fix(companion): clear stale Locator/Live Share/favourite refs on delete

Deleting a waypoint left the Locator pointed at coordinates that no longer
existed (it's a coordinate snapshot, so nothing noticed). Removing a contact
was worse: nothing cleared its favourite slot, its Locator/Live Share target,
or its per-contact mute/melody entry, so all four kept referencing a pubkey
that no longer resolved to anything.

- WaypointsView's Delete now calls UITask::clearTargetIfWaypoint() first.
- New AbstractUITask::onContactRemoved() hook, called from MyMesh.cpp's
  CMD_REMOVE_CONTACT handler, clears the favourite slot, the Locator target,
  and dm_notif/dm_melody entries for that pubkey. Live Share's DM target
  turns auto-share off instead of guessing a new recipient.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/ui-new/WaypointsView.h

@@ -269,6 +269,8 @@ public:
           }
         } else if (sel == 1) {                            // Delete
           if (wi >= 0 && wi < _task->waypoints().count()) {
+            const Waypoint& w = _task->waypoints().at(wi);
+            _task->clearTargetIfWaypoint(w.lat_1e6, w.lon_1e6);   // don't leave the Locator pointed at a deleted spot
             _task->waypoints().remove(wi);
             _task->saveWaypoints();
             if (_sel >= wpListCount()) _sel = wpListCount() - 1;