Add ringtone editor, buzzer volume, unread fixes and buzzer notification fixes

- RingtoneEditorScreen: 32-note step sequencer accessible via new Tools page on home screen. U/D=pitch, ENTER=octave cycle, L/R=cursor, MENU=options (play, duration, BPM, insert/delete, save). Notes stored packed in NodePrefs and persisted via DataStore.
- ToolsScreen: new "Tools" home page entry launching the editor
- Buzzer volume: 0-4 bar control in Settings > Sound, persisted to prefs
- Buzzer notifications: fixed notify() firing inside wrong serial-guard branch and before addChannelMsg set the channel index
- Unread counter: fixed newest-first index direction; watermark formula now correct
- OLED brightness: wider range via pre-charge register 0xD9 combined with contrast

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

examples/companion_radio/ui-new/UITask.h

@@ -55,6 +55,8 @@ class UITask : public AbstractUITask {
   UIScreen* home;
   UIScreen* settings;
   UIScreen* quick_msg;
+  UIScreen* tools_screen;
+  UIScreen* ringtone_edit;
   UIScreen* curr;
 
   void userLedHandler();
@@ -84,6 +86,11 @@ public:
   void gotoHomeScreen() { setCurrScreen(home); }
   void gotoSettingsScreen();
   void gotoQuickMsgScreen();
+  void gotoToolsScreen();
+  void gotoRingtoneEditor();
+  void playMelody(const char* melody);
+  void stopMelody();
+  bool isMelodyPlaying();
   void showAlert(const char* text, int duration_millis);
   void addChannelMsg(uint8_t channel_idx, const char* text) override;
   int  getMsgCount() const { return _msgcount; }