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-new/UITask.h

@@ -60,10 +60,11 @@ class UITask : public UITaskBase, public UiCoreHost {
   GenericVibration vibration;
 #endif
   unsigned long _next_refresh, _auto_off;
-  bool _display_was_on = true;   // loop(): the display back on and unlocked
+  bool _display_was_on = true;   // loop(): messagesInView() last time round
   NodePrefs* _node_prefs;
   bool _locked;
   bool _tool_from_home = false;
+  bool messagesInView() const;   // Messages on a lit, unlocked display
   unsigned long _lock_wake_until;  // when to blank screen again after locked wake (5s)
   int  _lock_seq_count;            // Enter presses while Back held (lock/unlock sequence)
   unsigned long _lock_seq_ms;      // millis() of last lock-sequence press (for timeout)