Bot: DM+channel modes with separate replies; buzzer auto-mute; UX fixes

- Bot redesign: DM bot active for all private messages (no pubkey filter),
  channel bot optional with separate enable flag; both can run simultaneously
- Bot: one shared trigger, separate reply texts for DM and channel
- Buzzer: add Auto mode (mutes when BT connected, unmutes on disconnect);
  per-channel force-on overrides still work as intended
- Settings: defer flash writes to menu exit (only when dirty)
- Settings: add Seconds toggle (hide clock seconds, refresh drops 1s→60s)
- Fullscreen msg preview: fix truncation (fmsg buf 79→140 chars)
- RingtoneEditor: fix melody playback cut after first note (RTTTL lib holds
  pointer — moved play buffer to member variable)
- BotScreen: remove key hint footer
- .gitattributes: protect README.md from upstream merge conflicts

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

examples/companion_radio/MyMesh.cpp

@@ -876,11 +876,11 @@ MyMesh::MyMesh(mesh::Radio &radio, mesh::RNG &rng, mesh::RTCClock &rtc, SimpleMe
   _prefs.ringtone_len = 0;       // no custom ringtone by default
   _prefs.home_pages_mask = 0x01FF;  // all pages visible
   _prefs.bot_enabled = 0;
+  _prefs.bot_channel_enabled = 0;
   _prefs.bot_channel_idx = 0;
   _prefs.bot_trigger[0] = '\0';
-  _prefs.bot_reply[0] = '\0';
-  _prefs.bot_target_type = 0;
-  memset(_prefs.bot_dm_pubkey, 0, sizeof(_prefs.bot_dm_pubkey));
+  _prefs.bot_reply_dm[0] = '\0';
+  _prefs.bot_reply_ch[0] = '\0';
   _prefs.auto_off_secs = 15;    // 15 seconds auto-off by default
   _prefs.tz_offset_hours = 0;  // UTC by default
   _prefs.low_batt_mv = 3400;  // auto-shutdown at 3.4V by default