Fix buzzer notifications, unread tracking, OLED dimming; remove dead code

- MyMesh: notify() now always fires for channel/contact messages regardless
  of serial connection; called after addChannelMsg so channel index is set
- ChannelHist: fix unread watermark direction (histEntry is newest-first,
  index 0=newest); counter now decrements correctly as user reads messages
- ChannelHist: reset unread to 0 after sending (user is active in channel)
- SH1106Display: add pre-charge register (0xD9) to setBrightness for wider
  dimming range; re-apply contrast+precharge in turnOn(); tune curve
- UITask: remove MsgPreviewScreen dead code (~180 lines, never shown)
- UITask: fix strcpy→snprintf in showAlert()

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

examples/companion_radio/MyMesh.cpp

@@ -465,9 +465,7 @@ void MyMesh::queueMessage(const ContactInfo &from, uint8_t txt_type, mesh::Packe
   bool should_display = txt_type == TXT_TYPE_PLAIN || txt_type == TXT_TYPE_SIGNED_PLAIN;
   if (should_display && _ui) {
     _ui->newMsg(path_len, from.name, text, offline_queue_len, from.type);
-    if (!_serial->isConnected()) {
-      _ui->notify(UIEventType::contactMessage);
-    }
+    _ui->notify(UIEventType::contactMessage);
   }
 #endif
 }
@@ -561,13 +559,10 @@ void MyMesh::onChannelMessageRecv(const mesh::GroupChannel &channel, mesh::Packe
     uint8_t frame[1];
     frame[0] = PUSH_CODE_MSG_WAITING; // send push 'tickle'
     _serial->writeFrame(frame, 1);
-  } else {
-#ifdef DISPLAY_CLASS
-    if (_ui) _ui->notify(UIEventType::channelMessage);
-#endif
   }
 #ifdef DISPLAY_CLASS
   if (_ui) _ui->addChannelMsg(channel_idx, text);
+  if (_ui) _ui->notify(UIEventType::channelMessage);
   const char *channel_name = "Unknown";
   ChannelDetails channel_details;
   if (getChannel(channel_idx, channel_details)) {