add a device message when backup transfer ends

that way, UI can just close the transfer dialog,
so that, at the end, both devices end in the chatlist.

we can also use this for troubleshooting -
if the device message is not present,
transfer did not succeed completely.

(a separate device message may be nice in that case -
but that is another effort,
same for making the device message reappear after deletion
or after some time)

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## 1.112.6 - unreleased
 
+### Changes
+
+- Add a device message after backup transfer #4301
+
 ### Fixed
 
 - Updated `iroh` from 0.4.0 to 0.4.1 to fix transfer of large accounts with many blob files.