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.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;