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.

rust/core/src/models.rs

@@ -466,6 +466,11 @@ pub struct IrcConfig {
     pub nick_serv_identify: bool,
     pub nick_serv_account: String,
     pub nick_serv_mode: SecretMode,
+    /// Log in with SASL PLAIN during registration (needs the network to offer it).
+    pub sasl: bool,
+    /// SASL account (defaults to the nick).
+    pub sasl_account: String,
+    pub sasl_mode: SecretMode,
     #[serde(deserialize_with = "nd")]
     pub auto_join_rooms: Vec<String>,
     pub ssh_tunnel: bool,