Share the ADMIN text and KILL reason, and test the review's gaps

ADMIN's location and email lines come from service.AdminLines, and
KillUser applies the default reason, so both transports send the same
reply. The Disconnect test now allows a quarter of one write window,
so a Disconnect that writes before returning fails it. QueryUserMode
and UserhostReply are tested with a read that fails. The handler tests
again cover USERHOST for a nick with no session, MODE +o on your own
nick, and the status of a successful KILL. handleMode's comment says
what it does.

Model: opus-5-5

internal/handlers/api.go

@@ -1951,8 +1951,9 @@ func (hdlr *Handlers) handleQuit(
 		http.StatusOK)
 }
 
-// handleMode handles the MODE command for channels and
-// users. Currently supports query-only (no mode changes).
+// handleMode handles the MODE command: a target that starts
+// with # is a channel, handled by handleChannelMode, and any
+// other target a nick, handled by handleUserMode.
 func (hdlr *Handlers) handleMode(
 	writer http.ResponseWriter,
 	request *http.Request,