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/NodePrefs.h

@@ -45,4 +45,7 @@ struct NodePrefs {  // persisted to file
   uint64_t ch_notif_muted;     // bitmask: bit i = channel i muted (only if override bit set)
   uint8_t  dm_show_all;        // 0=favourites only (default), 1=all chat contacts
   uint8_t  room_fav_only;      // 0=all room servers (default), 1=favourites only
+  uint8_t  ringtone_bpm_idx;   // index into {60,90,120,150,180}
+  uint8_t  ringtone_len;        // number of notes in custom ringtone (0 = use default)
+  uint8_t  ringtone_notes[32]; // packed: bits0-2=pitch, bits3-4=octave-4, bits5-6=dur_idx
 };