fix read state in self-chat and stop leaking read-marker into it (#18)

Self-chat never showed Read: the outgoing message was promoted by
sendDisplayedMarkerIfNeeded, but a later XEP-0184 <received/> receipt
or a LMM correction overwrote .read with .delivered/.sent directly,
bypassing Delivery.merged. Mark self-addressed outgoing messages read
as soon as the server accepts them and merge every weaker status into
the ladder instead of overwriting.

The self-addressed read-marker carried a plaintext <body>, so clients
that do not know our namespace (Conversations, Monal) rendered it as a
"Сообщение прочитано" bubble in the self-chat. Service stanzas now ride
on their payload element only.

Refs #18

Sources/Shared/XMPP/XMPPService.swift

@@ -829,7 +829,10 @@ final class XMPPService {
         message.to = JID(account.normalizedJID)
         message.type = .chat
         message.id = marker.id
-        message.body = Self.readMarkerBody
+        // No <body>: this is a service stanza for the user's own devices only.
+        // A plaintext body would leak into the self-chat and be rendered as a
+        // normal message by clients that do not know our read-marker namespace
+        // (Conversations, Monal), so the marker rides solely on its payload.
         addOriginID(marker.id, to: message)
         message.addChild(ReadStateSync.payloadElement(marker: marker))
         client.context.writer.write(message, writeCompleted: nil)
@@ -851,8 +854,6 @@ final class XMPPService {
         client.context.writer.write(message, writeCompleted: nil)
     }
 
-    private static let readMarkerBody = "Сообщение прочитано"
-
     /// Detects OMEMO 2 (`urn:xmpp:omemo:2`) payloads, which the pinned
     /// MartinOMEMO version cannot decrypt (it only speaks the legacy
     /// `eu.siacs.conversations.axolotl` namespace).