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>

src/helpers/ui/buzzer.cpp

@@ -44,6 +44,10 @@ void genericBuzzer::setVolume(uint8_t level) {
     if (isPlaying()) applyVolume();
 }
 
+void genericBuzzer::stop() {
+    rtttl::stop();
+}
+
 void genericBuzzer::loop() {
     if (!rtttl::done()) {
         rtttl::play();

src/helpers/ui/buzzer.h

@@ -30,6 +30,7 @@ class genericBuzzer
         bool isQuiet();  // get buzzer state on/off
         void setVolume(uint8_t level);  // 0=min..4=max
         uint8_t getVolume() const { return _volume_level; }
+        void stop();  // stop any playing melody
 
     private:
         uint8_t _volume_level = 4;