feat(irc): SASL PLAIN login and IRCv3 server-time

Registration now opens with CAP LS 302 (servers without IRCv3 ignore it
and register as before). From the offered capabilities (multi-line LS
lists included) the client requests server-time and, when the profile
enables it, sasl; then AUTHENTICATE PLAIN with the account (defaulting
to the nick) and a stored SASL password (irc-sasl-password), base64 in
400-byte chunks, and CAP END on success or failure — failures are shown
on the Server tab and the connection carries on (NickServ still works).
server-time tags stamp events with the server's clock, so replayed
history shows real times.

Tested against the fake IRC server: multi-line LS, REQ/ACK, the PLAIN
exchange, 903, CAP END, and a tagged message's timestamp.

docs/overview.md

@@ -168,6 +168,7 @@ Use Nexus Terminal when you need to:
 
 ### IRC capabilities
 
+- IRCv3 capability negotiation with SASL PLAIN login and server-time timestamps.
 - Nick, real name, TLS and an optional server password.
 - NickServ identification on connect.
 - Auto-join rooms.