feat: DM per-contact notification settings and mark-as-read

Long-press ENTER in the DM contact list opens a context menu with:
- "Mark as read": clears unread counter for that contact
- "Notif: default/OFF/ON": cycles notification state (same as channels)

Notification state is persisted in NodePrefs (dm_notif[16] table,
keyed by 4-byte pub_key prefix). When muted, the buzzer is silenced
for that contact's messages; when force-on, buzzer plays even in
quiet mode. Default follows the global buzzer setting.

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

examples/companion_radio/ui-new/UITask.h

@@ -38,6 +38,8 @@ class UITask : public AbstractUITask {
   int _msgcount;
   int _room_unread;
   int _last_notif_ch_idx;
+  uint8_t _last_notif_dm_prefix[4];
+  bool _last_notif_dm_valid;
   struct DMUnreadEntry { uint8_t prefix[4]; uint8_t count; };
   static const int DM_UNREAD_TABLE_SIZE = 16;
   DMUnreadEntry _dm_unread_table[DM_UNREAD_TABLE_SIZE];
@@ -81,6 +83,8 @@ public:
     _batt_mv = 0;
     _msgcount = _room_unread = 0;
     _last_notif_ch_idx = -1;
+    _last_notif_dm_valid = false;
+    memset(_last_notif_dm_prefix, 0, sizeof(_last_notif_dm_prefix));
     memset(_dm_unread_table, 0, sizeof(_dm_unread_table));
     curr = NULL;
   }