feat(server): restructure Pomodoro server into modular components

- move server logic to cmd/server/main.go
- create packages for websocket, pomodoro and broadcast handling
- define models for messages
- remove old GoTomato.go file
- update README

🤖

internal/pomodoro/pomodoro.go

@@ -0,0 +1,39 @@
+package pomodoro
+
+import (
+	"github.com/gorilla/websocket"
+	"sync"
+)
+
+const (
+	workDuration       = 15 * 60
+	shortBreakDuration = 5 * 60
+	longBreakDuration  = 10 * 60
+	sessions           = 4
+)
+
+var mu sync.Mutex // to synchronize access to shared state
+
+// RunPomodoroTimer iterates the Pomodoro work/break sessions.
+func RunPomodoroTimer(clients map[*websocket.Conn]bool) {
+	mu.Lock()
+	timerRunning = true
+
+	for session := 1; session <= sessions; session++ {
+		if !startTimer(clients, workDuration, "Work", session) {
+			break
+		}
+		if session == sessions {
+			if !startTimer(clients, longBreakDuration, "LongBreak", session) {
+				break
+			}
+		} else {
+			if !startTimer(clients, shortBreakDuration, "ShortBreak", session) {
+				break
+			}
+		}
+	}
+
+	timerRunning = false
+	mu.Unlock()
+}