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_test.go

@@ -420,6 +420,23 @@ func TestDefaultChromiumUserAgent_Content(t *testing.T) {
 	}
 }
 
+func TestChromiumUserAgent_MatchesVersion(t *testing.T) {
+	ua := chromiumUserAgent("136.0.7103.25")
+	if !strings.Contains(ua, "Chrome/136.0.0.0") {
+		t.Fatalf("expected Chrome/136.0.0.0 in UA, got %s", ua)
+	}
+	if strings.Contains(ua, "131") {
+		t.Fatalf("UA must not contain old hardcoded version 131: %s", ua)
+	}
+}
+
+func TestChromiumUserAgent_MajorOnly(t *testing.T) {
+	ua := chromiumUserAgent("140")
+	if !strings.Contains(ua, "Chrome/140.0.0.0") {
+		t.Fatalf("expected Chrome/140.0.0.0 in UA, got %s", ua)
+	}
+}
+
 // --- Viewport and UA defaults via mergeOptions ---
 
 func TestMergeOptions_DefaultViewport(t *testing.T) {