refactor(ui): share single KeyboardWidget instance across screens

Moves KeyboardWidget from a per-screen value member to a single instance
owned by UITask, passed to SettingsScreen, QuickMsgScreen, and BotScreen
as a pointer. Only one screen is ever active at a time, so the shared
state is safe. Saves ~1.5 KB of always-resident heap.

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

examples/companion_radio/ui-new/UITask.h

@@ -24,6 +24,7 @@
 #include "../NodePrefs.h"
 #include "../Trail.h"
 #include "../Waypoint.h"
+#include "KeyboardWidget.h"
 
 class UITask : public AbstractUITask {
   DisplayDriver* _display;
@@ -43,6 +44,7 @@ class UITask : public AbstractUITask {
   bool _lock_seq_used;             // true = suppress next back_btn CLICK (post-sequence release)
   char _alert[80];
   char _notif_mel_buf[220];  // persistent RTTTL buffer for custom notification melodies
+  KeyboardWidget _kb;        // shared across all screens — only one active at a time
   unsigned long _alert_expiry;
   int _msgcount;
   int _room_unread;