Count sent messages at send time; implement first-contact greeting

- messages_sent now increments when a message is queued (async_send_message),
  not only when a delivery proof returns. LXMessage's delivery callback only
  fires on a returned proof (LXMessage.py ~566), which many peers/methods never
  send, so the "Messages sent" sensor was stuck at 0 for messages that were
  actually delivered. The delivery callback now only emits the event.
- Implement the previously-inert "greeting" option: send it automatically the
  first time each sender contacts HA (tracked per sender in RuntimeData).
- Clarify the notify entity (rename to "Send message" / "Отправить сообщение")
  and document greeting + notify in strings/translations/README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

custom_components/reticulum/manifest.json

@@ -12,5 +12,5 @@
   "loggers": ["RNS", "LXMF"],
   "requirements": ["rns>=0.9.0", "lxmf>=0.6.0"],
   "single_config_entry": true,
-  "version": "1.0.5"
+  "version": "1.0.6"
 }