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>

browser_init.go

@@ -52,15 +52,11 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		return nil, ErrInvalidBrowserSelection
 	}
 
-	// Auto-select a User-Agent matching the browser engine when the caller hasn't set one.
-	if opt.UserAgent == "" {
-		switch opt.Browser {
-		case BrowserChromium:
-			opt.UserAgent = DefaultChromiumUserAgent
-		default:
-			opt.UserAgent = DefaultFirefoxUserAgent
-		}
-	}
+	// User-Agent auto-selection is deferred until after browser launch so we
+	// can read the real browser version and build a UA that matches the
+	// sec-ch-ua header Chromium sends automatically. A mismatched version
+	// (e.g. UA says Chrome/131 while sec-ch-ua says Chromium/136) is a
+	// well-known bot-detection signal that causes 403s on many sites.
 
 	// Collect launch args and init scripts, starting with any stealth-mode presets.
 	stealth := opt.Stealth == nil || *opt.Stealth
@@ -117,6 +113,16 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 		}
 	}
 
+	// Auto-select User-Agent now that we know the real browser version.
+	if opt.UserAgent == "" {
+		switch opt.Browser {
+		case BrowserChromium:
+			opt.UserAgent = chromiumUserAgent(browser.Version())
+		default:
+			opt.UserAgent = DefaultFirefoxUserAgent
+		}
+	}
+
 	var viewport *playwright.Size
 	if opt.Dimensions.Width > 0 && opt.Dimensions.Height > 0 {
 		viewport = &playwright.Size{