refactor(capture): screen capture in subprocess

- Move screen capture to child process
- Isolate GDI/BitBlt crashes from agent
- Use -capture flag for internal helper mode
- Add protocol for frame request/response
- Make monitor enumeration thread-safe

lib/agent/handlers.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"time"
 
+	"tea.chunkbyte.com/kato/go-worm/lib/capture"
 	"tea.chunkbyte.com/kato/go-worm/lib/command"
 	"tea.chunkbyte.com/kato/go-worm/lib/config"
 	"tea.chunkbyte.com/kato/go-worm/lib/files"
@@ -329,9 +330,9 @@ func (a *Agent) handleScreenshot(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 	}
-	frame, err := screenshot.CaptureMonitor(monitor, format, quality)
+	frame, err := capture.CaptureMonitor(monitor, format, quality)
 	if err != nil {
-		if errors.Is(err, screenshot.ErrMonitorNotFound) {
+		if errors.Is(err, capture.ErrMonitorNotFound) {
 			helpers.WriteError(w, http.StatusBadRequest, err.Error())
 			return
 		}