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/app/src/profile_editor.rs

@@ -702,6 +702,7 @@ impl ProfileEditor {
         });
         section(ui, "Devices", |ui| {
             check(ui, "Play Remote Audio", &mut c.audio, "");
+            check(ui, "Redirect Printer", &mut c.redirect_printer, "Adds a \"Nexus Terminal\" printer; jobs are saved as PostScript files in Downloads");
             if cfg!(unix) {
                 label(ui, "Shared Folder");
                 ui.horizontal(|ui| {