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/reticulum_client.py

@@ -362,9 +362,15 @@ class ReticulumManager:
     ) -> str:
         """Send an LXMF message. Returns the message hash (hex)."""
         dest_hash = self._normalise_hash(destination)
-        return await self.hass.async_add_executor_job(
+        message_hash = await self.hass.async_add_executor_job(
             self._send, dest_hash, content, title, method, fields
         )
+        # Count at send time. The delivery callback only fires on a returned
+        # proof, which many peers/methods never send, so counting there would
+        # leave this stuck at 0 even for messages that were delivered fine.
+        self.state.messages_sent += 1
+        self._push_state()
+        return message_hash
 
     def _send(
         self,
@@ -535,10 +541,10 @@ class ReticulumManager:
 
     @callback
     def _on_outbound_delivered(self, message: Any) -> None:
+        # messages_sent is counted at send time (see async_send_message); this
+        # callback only confirms delivery, so it just emits the event.
         import RNS  # noqa: PLC0415
 
-        self.state.messages_sent += 1
-        self._push_state()
         _fire_event(
             self.hass,
             "delivered",