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/secrets.rs

@@ -35,12 +35,13 @@ pub enum SecretKind {
     RdpSshPassword,
     RdpSshPassphrase,
     RdpGatewayPassword,
+    IrcSaslPassword,
 }
 
 impl SecretKind {
     /// Every kind, so deleting a profile can purge them all from the keychain. A new variant
     /// must be added here as well as to `as_str`.
-    pub const ALL: [SecretKind; 16] = [
+    pub const ALL: [SecretKind; 17] = [
         Self::Password,
         Self::Passphrase,
         Self::Totp,
@@ -57,6 +58,7 @@ impl SecretKind {
         Self::RdpSshPassword,
         Self::RdpSshPassphrase,
         Self::RdpGatewayPassword,
+        Self::IrcSaslPassword,
     ];
 
     /// The storage name, shared with the Go app.
@@ -78,6 +80,7 @@ impl SecretKind {
             Self::RdpSshPassword => "rdp-ssh-password",
             Self::RdpSshPassphrase => "rdp-ssh-passphrase",
             Self::RdpGatewayPassword => "rdp-gateway-password",
+            Self::IrcSaslPassword => "irc-sasl-password",
         }
     }