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

README.md

@@ -249,9 +249,11 @@ what channel bans (`+b` mode) match against.
   hold the same nick simultaneously.
 - Nicks are **case-sensitive** (unlike traditional IRC). `Alice` and `alice` are
   different nicks.
-- Nick length: 1–32 characters. Over the HTTP API a nick starts with a letter or
-  `_`, followed by letters, digits and `` _-[]\^{}|` ``. The IRC listener cuts a
-  longer nick to 32 bytes and checks nothing else.
+- Nick length: 1–32 characters. A nick starts with a letter or `_`, followed by
+  letters, digits and `` _-[]\^{}|` ``. The HTTP API and the IRC listener both
+  apply this rule. On the IRC listener, a `NICK` with any other nick, before or
+  after registration, gets ERR_ERRONEUSNICKNAME (432) and leaves the nick as it
+  was.
 - Nicks are **released when a session is destroyed** (via `QUIT` command or
   session expiry). There is no nick registration or reservation system.
 - Nick changes are broadcast to all users sharing a channel with the changer, as