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/service/service.go

@@ -44,6 +44,18 @@ var validChannelRe = regexp.MustCompile(
 	`^#[a-zA-Z0-9_\-]{1,63}$`,
 )
 
+// validNickRe matches the nicks both transports accept: a
+// letter or _, then up to 31 letters, digits or
+// _-[]\^{}|`.
+var validNickRe = regexp.MustCompile(
+	`^[a-zA-Z_][a-zA-Z0-9_\-\[\]\\^{}|` + "`" + `]{0,31}$`,
+)
+
+// ValidNick reports whether validNickRe matches nick.
+func ValidNick(nick string) bool {
+	return validNickRe.MatchString(nick)
+}
+
 // Params defines the dependencies for creating a Service.
 type Params struct {
 	fx.In