feat(04-01): add Notifier interface and FocusTimer

- Notifier interface with ExecNotifier (notify-send) and NullNotifier
- FocusTimer thread-safe with Set/IsActive/Remaining
- shortName helper for session display names
- Full test coverage for all components

focus_test.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+func TestFocusTimer_Set(t *testing.T) {
+	var ft FocusTimer
+	ft.Set(30 * time.Minute)
+	if !ft.IsActive() {
+		t.Error("IsActive() = false after Set(30min), want true")
+	}
+}
+
+func TestFocusTimer_Expired(t *testing.T) {
+	var ft FocusTimer
+	ft.Set(0)
+	if ft.IsActive() {
+		t.Error("IsActive() = true after Set(0), want false")
+	}
+}
+
+func TestFocusTimer_Remaining(t *testing.T) {
+	var ft FocusTimer
+	ft.Set(30 * time.Minute)
+	rem := ft.Remaining()
+	if rem <= 0 {
+		t.Errorf("Remaining() = %v, want > 0", rem)
+	}
+	if rem > 30*time.Minute {
+		t.Errorf("Remaining() = %v, want <= 30min", rem)
+	}
+}
+
+func TestFocusTimer_ZeroValue(t *testing.T) {
+	var ft FocusTimer
+	if ft.IsActive() {
+		t.Error("zero-value FocusTimer IsActive() = true, want false")
+	}
+	if ft.Remaining() != 0 {
+		t.Errorf("zero-value Remaining() = %v, want 0", ft.Remaining())
+	}
+}