fix: derive Chromium User-Agent from actual browser version

The hardcoded DefaultChromiumUserAgent said Chrome/131 while the
installed Chromium was v136. Chromium's sec-ch-ua header is generated
from the real engine version, so sites comparing User-Agent against
sec-ch-ua detected the mismatch as bot traffic and returned 403.

Now the User-Agent is built after browser launch using browser.Version(),
keeping the Chrome/N token in sync with sec-ch-ua's Chromium;v="N".

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

stealth.go

@@ -3,6 +3,7 @@ package extractor
 import (
 	"fmt"
 	"math/rand/v2"
+	"strings"
 )
 
 // stealthChromiumArgs are launch arguments that reduce automation detection for Chromium-based browsers.
@@ -269,3 +270,20 @@ func buildFirefoxStealthScripts(p firefoxHWProfile) []string {
 	})`,
 	}
 }
+
+// chromiumUserAgent builds a Chromium User-Agent string from the actual
+// browser version. This keeps the UA in sync with the sec-ch-ua header
+// that Chromium sends automatically, avoiding a version mismatch that
+// anti-bot systems use to detect automation.
+func chromiumUserAgent(version string) string {
+	// version is typically "136.0.7103.25"; we need the major for the
+	// Chrome/MAJ.0.0.0 token and the full version for the template.
+	major := version
+	if i := strings.IndexByte(version, '.'); i > 0 {
+		major = version[:i]
+	}
+	return fmt.Sprintf(
+		"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/%s.0.0.0 Safari/537.36",
+		major,
+	)
+}