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

notify.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"context"
+	"os/exec"
+	"path/filepath"
+	"time"
+)
+
+// Notifier sends desktop notifications.
+type Notifier interface {
+	Notify(title, body string) error
+}
+
+// ExecNotifier calls notify-send via os/exec.
+type ExecNotifier struct{}
+
+func (n *ExecNotifier) Notify(title, body string) error {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	return exec.CommandContext(ctx, "notify-send",
+		"--urgency=critical",
+		"--app-name=vmux",
+		title,
+		body,
+	).Run()
+}
+
+// NullNotifier discards all notifications (for tests and focus mode).
+type NullNotifier struct{}
+
+func (n *NullNotifier) Notify(_, _ string) error {
+	return nil
+}
+
+// shortName returns a human-readable short name for a session.
+// Prefers Label if set, otherwise uses the last component of Cwd.
+func shortName(s SessionInfo) string {
+	if s.Label != "" {
+		return s.Label
+	}
+	return filepath.Base(s.Cwd)
+}