feat: melody assignment per channel/DM, dual ringtone slots, volume cleanup

Add second ringtone slot (ringtone2_*) to NodePrefs and DataStore.
Add per-channel and per-DM melody assignment (built-in / melody 1 / 2)
stored in NodePrefs bitmasks and DmMelodyEntry table.
Settings screen exposes DM/CH sound items; RingtoneEditorScreen supports
switching between slots. notify() uses buildMelodyFromPrefs() to select
the correct RTTTL string per contact.

Remove broken nRF52 volume control from buzzer.cpp: tone() uses
NRF_PWM2 with DECODER.MODE=Auto so TASKS_NEXTSTEP has no effect and
duty cannot be changed before the first DMA read without patching the
Arduino core. applyVolume() is now a no-op on nRF52.

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

src/helpers/ui/buzzer.cpp

@@ -32,11 +32,16 @@ bool genericBuzzer::isPlaying() {
 }
 
 void genericBuzzer::applyVolume() {
+#if !defined(NRF52_PLATFORM)
     // After tone() sets 50% duty, analogWrite overrides duty cycle on the same PWM channel.
     // Level 4 = 50% (leave tone as-is), lower levels reduce duty = quieter output.
     static const uint8_t duty[] = { 6, 20, 50, 90, 128 };
     uint8_t d = duty[_volume_level < 5 ? _volume_level : 4];
     if (d < 128) analogWrite(PIN_BUZZER, d);
+#endif
+    // On nRF52, tone() uses NRF_PWM2 with DECODER.MODE=Auto. The DMA reads duty at SEQSTART
+    // before software can write, and TASKS_NEXTSTEP only works in NextStep mode — so duty
+    // cannot be changed mid-note without patching the Arduino core. Volume has no effect.
 }
 
 void genericBuzzer::setVolume(uint8_t level) {