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>

examples/companion_radio/ui-new/UITask.h

@@ -34,6 +34,7 @@ class UITask : public AbstractUITask {
   unsigned long _next_refresh, _auto_off;
   NodePrefs* _node_prefs;
   char _alert[80];
+  char _notif_mel_buf[220];  // persistent RTTTL buffer for custom notification melodies
   unsigned long _alert_expiry;
   int _msgcount;
   int _room_unread;
@@ -99,7 +100,7 @@ public:
   void gotoSettingsScreen();
   void gotoQuickMsgScreen();
   void gotoToolsScreen();
-  void gotoRingtoneEditor();
+  void gotoRingtoneEditor(int slot = 0);
   void gotoBotScreen();
   void gotoNearbyScreen();
   void gotoDashboardConfig();