Initial commit: split VNOX monorepo into VNOX-Server

Unified server binary (vnox-serverd) combining:
- Gateway (TCP, auth, channels, sessions, SQLite)
- Voice-node (UDP relay, Opus, jitter buffer)

Standalone gateway (vnox-gateway) and voice-node (vnox-voice-node) preserved as sub-crates.

gateway/src/handler/channel/mod.rs

@@ -0,0 +1,33 @@
+pub mod create;
+pub mod join;
+pub mod leave;
+
+pub use create::{handle_channel_create, handle_channel_delete, handle_channel_list};
+pub use join::join;
+pub use leave::leave;
+
+use crate::{
+    net::state::{BroadcastMsg, State},
+    proto::{PacketId, UserLeavePayload, encode_packet, to_payload},
+};
+
+pub async fn broadcast_leave(state: &State, channel_id: &str, session_id: &str) {
+    if let Some(sess) = crate::domain::session::get(&state.sessions, session_id).await {
+        let p = UserLeavePayload {
+            channel_id: channel_id.into(),
+            user_id: sess.user_id.clone(),
+        };
+        let _ = state.broadcast.send(BroadcastMsg {
+            channel_id: Some(channel_id.into()),
+            exclude_session: Some(session_id.into()),
+            target_session_id: None,
+            data: encode_packet(PacketId::UserLeave, 0, &to_payload(&p)),
+        });
+    }
+}
+
+async fn set_channel(state: &State, session_id: &str, ch: Option<String>) {
+    if let Some(s) = state.sessions.write().await.get_mut(session_id) {
+        s.channel_id = ch;
+    }
+}