feat: initial gosleep-timer implementation

Go TUI sleep timer with modular architecture.
- Config: YAML-based, profile inheritance, auto WM detection
- Engine: timer, executor, stage pipeline (pre→timer→post)
- Modules: workspace (niri/hyprland/kde), media, lock, kill,
  notify, sound, brightness, mute, custom, script
- TUI: bubbletea app with profiles, module toggles, progress bar
- CLI: run, list-profiles, export, import, history, stats
- History: JSONL log with statistics
- QR: config export as ASCII QR

modules/media/media.go

@@ -0,0 +1,40 @@
+package media
+
+import (
+	"fmt"
+	"os/exec"
+
+	"github.com/forust/gosleep-timer/config"
+	"github.com/forust/gosleep-timer/modules"
+)
+
+type MediaModule struct{}
+
+func (m *MediaModule) Name() string {
+	return "media"
+}
+
+func (m *MediaModule) Enabled(cfg *config.Modules) bool {
+	return cfg.Media != nil && cfg.Media.Enabled
+}
+
+func (m *MediaModule) BuildCmds(ctx *modules.ModuleContext, stage modules.Stage) []string {
+	if stage != modules.StagePre {
+		return nil
+	}
+	if ctx.Config.Media.Action == "none" {
+		return nil
+	}
+	return []string{fmt.Sprintf("playerctl %s", ctx.Config.Media.Action)}
+}
+
+func (m *MediaModule) Validate() error {
+	if _, err := exec.LookPath("playerctl"); err != nil {
+		return fmt.Errorf("media: playerctl not found")
+	}
+	return nil
+}
+
+func init() {
+	modules.Register(&MediaModule{})
+}