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/ui-new/UITask.h

@@ -53,7 +53,6 @@ class UITask : public AbstractUITask {
 
   UIScreen* splash;
   UIScreen* home;
-  UIScreen* msg_preview;
   UIScreen* settings;
   UIScreen* quick_msg;
   UIScreen* curr;