refactor: one place each for "in view" and the keyboard's backspace

ui-new's messagesInView() and ui-lvgl's threadInView() hold the
lit-and-unlocked test the viewing checks repeated; the keyboard's three
copies of the backspace call deleteBeforeCursor(). A stale comment on the
cursor-mode hint goes, onDisplayOn() sits after the view helpers it calls.

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

examples/companion_radio/ui-lvgl/UITask.cpp

@@ -1914,12 +1914,16 @@ void UITask::onMessageArrived(const UiEvent& ev) {
 }
 
 // On screen: a conversation left open under the lock or a dark display isn't read.
+bool UITask::threadInView() const {
+  return !_asleep && !locked() && _screen == SCR_THREAD;
+}
+
 bool UITask::isViewingChannel(uint8_t channel_idx) {
-  return !_asleep && !locked() && _screen == SCR_THREAD && _thread_is_channel && _thread_channel == channel_idx;
+  return threadInView() && _thread_is_channel && _thread_channel == channel_idx;
 }
 
 bool UITask::isViewingDM(const uint8_t* pub_key) {
-  return !_asleep && !locked() && _screen == SCR_THREAD && !_thread_is_channel && memcmp(_thread_key, pub_key, 4) == 0;
+  return threadInView() && !_thread_is_channel && memcmp(_thread_key, pub_key, 4) == 0;
 }
 
 // ── Status bar + toast (top layer, over every screen) ─────────────────────────

examples/companion_radio/ui-lvgl/UITask.h

@@ -430,6 +430,7 @@ private:
   void buildThread();
   void threadOpenAt(int first);
   void reopenThreadIfUnread();
+  bool threadInView() const;   // a thread on a lit, unlocked screen
   void refreshThread();
   void refreshThreadAges();
   uint32_t threadSignature() const;