Resize captures to 50% before serving

Uses sips to halve the image width, reducing transfer size and
downstream processing cost.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.go

@@ -12,7 +12,9 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -132,6 +134,16 @@ func captureHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Resize to 50% using sips
+	if out, err := exec.Command("sips", "-g", "pixelWidth", tmpFile).Output(); err == nil {
+		re := regexp.MustCompile(`pixelWidth:\s*(\d+)`)
+		if m := re.FindSubmatch(out); m != nil {
+			if w, _ := strconv.Atoi(string(m[1])); w > 0 {
+				exec.Command("sips", "--resampleWidth", strconv.Itoa(w/2), tmpFile).Run()
+			}
+		}
+	}
+
 	log.Printf("Captured display %s", display)
 	serveAndDelete(w, tmpFile)
 }