feat: add client representation and presence broadcasting

internal/collaboration/client.go

···
 package collaboration
 
 import (
+	"encoding/json"
+	"log"
+
 	"github.com/gorilla/websocket"
 )
 
···
 	Color   string
 	roomKey string
 }
+
+type ClientMessage struct {
+	Type    string          `json:"type"`
+	RKey    string          `json:"rkey,omitempty"`
+	DID     string          `json:"did,omitempty"`
+	Delta   json.RawMessage `json:"delta,omitempty"`
+	Cursor  *CursorPos      `json:"cursor,omitempty"`
+	Comment *CommentMsg     `json:"comment,omitempty"`
+}
+
+type CursorPos struct {
+	Position     int `json:"position"`
+	SelectionEnd int `json:"selectionEnd"`
+}
+
+type CommentMsg struct {
+	ParagraphID string `json:"paragraphId"`
+	Text        string `json:"text"`
+}
+
+type PresenceUser struct {
+	DID   string `json:"did"`
+	Name  string `json:"name"`
+	Color string `json:"color"`
+}
+
+type PresenceMessage struct {
+	Type  string         `json:"type"`
+	Users []PresenceUser `json:"users"`
+}
+
+func NewClient(hub *Hub, conn *websocket.Conn, did, name, color, roomKey string) *Client {
+	return &Client{
+		hub:     hub,
+		conn:    conn,
+		send:    make(chan []byte, 256),
+		DID:     did,
+		Name:    name,
+		Color:   color,
+		roomKey: roomKey,
+	}
+}
+
+func (c *Client) ReadPump() {
+	defer func() {
+		if room := c.hub.GetRoom(c.roomKey); room != nil {
+			room.unregister <- c
+		}
+		c.conn.Close()
+	}()
+	for {
+		_, message, err := c.conn.ReadMessage()
+		if err != nil {
+			if websocket.IsUnexpectedCloseError(err, websocket.CloseGoingAway, websocket.CloseAbnormalClosure) {
+				log.Printf("WebSocket error: %v", err)
+			}
+			break
+		}
+		_ = message
+	}
+}
+
+func (c *Client) WritePump() {
+	defer c.conn.Close()
+	for {
+		message, ok := <-c.send
+		if !ok {
+			c.conn.WriteMessage(websocket.CloseMessage, []byte{})
+			return
+		}
+		if err := c.conn.WriteMessage(websocket.TextMessage, message); err != nil {
+			return
+		}
+	}
+}

internal/collaboration/hub.go

···
 package collaboration
 
 import (
+	"encoding/json"
 	"sync"
 )
 
···
 	return room
 }
 
+func (h *Hub) GetRoom(rkey string) *Room {
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	return h.rooms[rkey]
+}
+
 func (r *Room) run() {
 	for {
 		select {
···
 			r.mu.Lock()
 			r.clients[client] = true
 			r.mu.Unlock()
-			r.broadcastPresence()
 		case client := <-r.unregister:
 			r.mu.Lock()
 			if _, ok := r.clients[client]; ok {
···
 				close(client.send)
 			}
 			r.mu.Unlock()
-			r.broadcastPresence()
 		case message := <-r.broadcast:
 			r.mu.RLock()
 			for client := range r.clients {
···
 	r.broadcast <- message
 }
 
+func (r *Room) GetPresence() []PresenceUser {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	users := make([]PresenceUser, 0, len(r.clients))
+	for client := range r.clients {
+		users = append(users, PresenceUser{
+			DID:   client.DID,
+			Name:  client.Name,
+			Color: client.Color,
+		})
+	}
+	return users
+}
+
 func (r *Room) broadcastPresence() {
-	// Implementation in Task 2.2
+	presence := PresenceMessage{
+		Type:  "presence",
+		Users: r.GetPresence(),
+	}
+	data, _ := json.Marshal(presence)
+	r.Broadcast(data)
 }