fix: prevent concurrent write to websocket connection

- refactor client handling and message broadcasting
- replace Client struct
- implement SendMessage method in Client struct for safer message sending
- update client map to use *models.Client instead of bool
- adjust BroadcastMessage and RunPomodoroTimer functions to use new client type

🤖

internal/pomodoro/pomodoro.go

@@ -24,7 +24,7 @@ var pomodoroResumeChannel = make(chan bool, 1)
 var mu sync.Mutex // to synchronize access to shared state
 
 // RunPomodoroTimer iterates the Pomodoro work/break sessions.
-func RunPomodoroTimer(clients map[*websocket.Conn]bool) {
+func RunPomodoroTimer(clients map[*websocket.Conn]*models.Client) {
 	mu.Lock()
 	pomodoroRunning = true
 	pomodoroPaused = false
@@ -51,7 +51,7 @@ func RunPomodoroTimer(clients map[*websocket.Conn]bool) {
 }
 
 // ResetPomodoro resets the running Pomodoro timer.
-func ResetPomodoro(clients map[*websocket.Conn]bool) {
+func ResetPomodoro(clients map[*websocket.Conn]*models.Client) {
 	mu.Lock()
 	pomodoroRunning = false // Reset the running state
 	pomodoroPaused = false  // Reset the paused state