Check nicks on the IRC listener with the HTTP API's rule (closes #142)

The nick rule moves from internal/handlers to internal/service as
ValidNick, and the HTTP API and both IRC listener NICK paths call it.
The IRC listener used to cut a nick to 32 bytes and accept anything
else, so a nick with spaces such as "bob PRIVMSG #inj :hi" made the
lines built from it read as another user's PRIVMSG. It now answers a
nick the rule refuses with ERR_ERRONEUSNICKNAME (432), before and
after registration, and keeps the nick as it was.

Model: opus-5-5

internal/handlers/api.go

@@ -19,10 +19,6 @@ import (
 	"sneak.berlin/go/neoirc/pkg/irc"
 )
 
-var validNickRe = regexp.MustCompile(
-	`^[a-zA-Z_][a-zA-Z0-9_\-\[\]\\^{}|` + "`" + `]{0,31}$`,
-)
-
 var validUsernameRe = regexp.MustCompile(
 	`^[a-zA-Z0-9_\-\[\]\\^{}|` + "`" + `]{1,32}$`,
 )
@@ -211,7 +207,7 @@ func (hdlr *Handlers) handleCreateSession(
 
 	payload.Nick = strings.TrimSpace(payload.Nick)
 
-	if !validNickRe.MatchString(payload.Nick) {
+	if !service.ValidNick(payload.Nick) {
 		hdlr.respondError(
 			writer, request,
 			"invalid nick format",
@@ -1551,7 +1547,7 @@ func (hdlr *Handlers) handleNick(
 
 	newNick := strings.TrimSpace(lines[0])
 
-	if !validNickRe.MatchString(newNick) {
+	if !service.ValidNick(newNick) {
 		hdlr.respondIRCError(
 			writer, request, clientID, sessionID,
 			irc.ErrErroneusNickname, nick, []string{newNick},