fix: polish lock screen sequence — hints update instantly, screen stays on, triple-Back blocked

- Turn display on at first Back+Enter press so hints are visible even when screen was dark
- Extend _lock_wake_until on every sequence step to prevent screen blanking mid-press
- Set _next_refresh=0 on every Enter press (not just on completion) for immediate hint redraw
- Add _lock_seq_used flag to suppress the Back CLICK that fires on release after unlock
- Block triple-Back (buzzer toggle) when screen is locked

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

examples/companion_radio/ui-new/UITask.h

@@ -37,6 +37,7 @@ class UITask : public AbstractUITask {
   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)
+  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
   unsigned long _alert_expiry;
@@ -92,7 +93,7 @@ public:
     _msgcount = _room_unread = 0;
     _locked = false;
     _lock_wake_until = 0;
-    _lock_seq_count = 0; _lock_seq_ms = 0;
+    _lock_seq_count = 0; _lock_seq_ms = 0; _lock_seq_used = false;
     _last_notif_ch_idx = -1;
     _last_notif_dm_valid = false;
     memset(_last_notif_dm_prefix, 0, sizeof(_last_notif_dm_prefix));