harness: add /screen page and /screen/ws WebSocket VNC relay

Reads /etc/hyperhive/gui.json at startup to get the VNC port written
by the weston-vnc ExecStart script (issue #50). Adds:
- gui_vnc_port: Option<u16> on AppState
- gui_enabled: bool on StateSnapshot (for issue #52 screen link)
- GET /screen: serves a minimal RFB-over-WebSocket viewer (screen.html)
- GET /screen/ws: upgrades to WebSocket and byte-pumps to 127.0.0.1:<vnc_port>

The relay is a pure two-task byte pump (WS→TCP and TCP→WS), transparent
to any RFB variant including VeNCrypt. Returns 404 when gui is not
enabled.

screen.html is a self-contained RFB client: handshake, FramebufferUpdate
(Raw encoding), pointer and keyboard forwarding — enough to display the
desktop and interact with it. noVNC assets (issue #52) replace this.

Closes #51

hive-ag3nt/Cargo.toml

@@ -9,6 +9,7 @@ workspace = true
 [dependencies]
 anyhow.workspace = true
 axum.workspace = true
+futures-util = "0.3"
 clap.workspace = true
 hive-fr0nt.workspace = true
 hive-sh4re.workspace = true