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/MyMesh.cpp

@@ -865,6 +865,8 @@ MyMesh::MyMesh(mesh::Radio &radio, mesh::RNG &rng, mesh::RTCClock &rtc, SimpleMe
   _prefs.gps_interval = 0;      // No automatic GPS updates by default
   _prefs.display_brightness = 2; // medium brightness by default
   _prefs.buzzer_volume = 4;      // max volume by default
+  _prefs.ringtone_bpm_idx = 2;  // 120 bpm default
+  _prefs.ringtone_len = 0;       // no custom ringtone by default
   _prefs.auto_off_secs = 15;    // 15 seconds auto-off by default
   _prefs.tz_offset_hours = 0;  // UTC by default
   _prefs.low_batt_mv = 3400;  // auto-shutdown at 3.4V by default