connection: bind the port-forward multiplex flag into the login scope digest

The hbb_common bump brings `PortForward.multiplex`, and the exhaustive
destructuring in login_scope_digest exists to force this decision on every
new field. A multiplexed tunnel takes its targets per channel rather than
from this login, so a retry that flips the flag asks for a different scope
than the one that was approved.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019aokqJuhjvB3kijXtAg5Ns

src/server/connection.rs

@@ -2576,11 +2576,13 @@ impl Connection {
                 let PortForward {
                     host,
                     port,
+                    multiplex,
                     special_fields: _,
                 } = pf;
                 push(b"port_forward");
                 push(host.as_bytes());
                 push(&port.to_le_bytes());
+                push(&[*multiplex as u8]);
             }
             // Variants this build does not know execute as remote, so they latch as remote.
             None | Some(_) => push(b"remote"),