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/strings.json

@@ -37,7 +37,7 @@
           "allow_all_senders": "Reply to any sender",
           "allowed_identities": "Allowed sender addresses",
           "default_recipient": "Default recipient address (for the notify entity)",
-          "greeting": "Greeting message",
+          "greeting": "First-contact greeting message",
           "delivery_method": "Default delivery method",
           "announce_interval": "Announce interval",
           "propagation_node": "Propagation node address",
@@ -48,6 +48,7 @@
           "enable_assist": "When enabled, incoming LXMF messages are answered by the Assist conversation agent and the reply is sent back over Reticulum.",
           "allowed_identities": "One address per line (or comma separated). Only used when 'Reply to any sender' is off.",
           "default_recipient": "16-byte hex LXMF address that the notify.reticulum entity sends to.",
+          "greeting": "Sent automatically the first time each sender messages Home Assistant (once per sender per restart). Leave blank to disable.",
           "announce_interval": "How often (seconds) to announce our address. 0 disables periodic announces.",
           "propagation_node": "Optional LXMF propagation node for store-and-forward delivery.",
           "sync_interval": "How often (seconds) to pull queued messages from the propagation node. 0 disables."
@@ -79,7 +80,7 @@
       "sync": { "name": "Sync propagation node" }
     },
     "notify": {
-      "message": { "name": "Message" }
+      "message": { "name": "Send message" }
     }
   },
   "selector": {