feat(rdp): Kerberos logon through a configured KDC

An RDP profile can name a Kerberos KDC; CredSSP then authenticates with
Kerberos instead of NTLM, reaching the KDC through a small TCP client
(RFC 4120 length-prefixed framing, 5 s connect / 10 s read timeouts,
replies capped at 1 MiB so a hostile KDC can't force a huge allocation).
UDP KDCs are refused with a clear message. Tested against a fake KDC
(round trip, oversized reply, UDP); not against a real realm.

Smart-card logon is not implemented: it needs PC/SC hardware and a card
to build and test against.

rust/core/src/models.rs

@@ -411,6 +411,8 @@ pub struct RdpConfig {
     pub allow_tls_only: bool,
     /// Play remote audio locally.
     pub audio: bool,
+    /// Log on with Kerberos through this KDC (`host` or `host:port`, TCP); empty uses NTLM.
+    pub kerberos_kdc: String,
     /// Local folder shared with the remote desktop as a drive (empty: none). Linux and macOS.
     pub shared_folder: String,
     /// Offer a printer; print jobs are saved as PostScript files in Downloads.