feat(rdp): printer redirection, saved as PostScript

An optional 'Nexus Terminal' printer (IronRDP's PostScript driver
announce) appears on the remote desktop; each job is written to
'Nexus print <time>.ps' in Downloads, with a toast saying where. Jobs
are capped at 256 MiB and empty jobs leave no file. The shared drive and
the printer now share one RDPDR backend (rdp_devices); the printer
works on every platform, the drive on Linux and macOS.

Tested with a unit test driving create/write/close; xrdp has no printer
redirection, so not exercised against a server.

rust/core/src/models.rs

@@ -413,6 +413,8 @@ pub struct RdpConfig {
     pub audio: bool,
     /// 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.
+    pub redirect_printer: bool,
     /// Trust policy for the server's TLS certificate (pinned like an SSH host key).
     #[serde(deserialize_with = "lenient")]
     pub host_key_policy: HostKeyPolicy,