fix: set default viewport for NewBrowser and align User-Agent with engine

NewBrowser previously had no viewport (strong headless signal) and used a
Firefox User-Agent unconditionally, even for Chromium instances (detectable
mismatch).

Add per-engine UA constants (DefaultFirefoxUserAgent, DefaultChromiumUserAgent)
and auto-select the matching UA in initBrowser when the caller hasn't set one
explicitly. Keep DefaultUserAgent as a backward-compatible alias.

Add 1920x1080 default viewport to NewBrowser (most common desktop resolution).
NewInteractiveBrowser keeps its existing 1280x720 default but also gains
engine-aware UA selection.

Closes #70

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

browser_init.go

@@ -52,6 +52,16 @@ 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
+		}
+	}
+
 	// Collect launch args and init scripts, starting with any stealth-mode presets.
 	stealth := opt.Stealth == nil || *opt.Stealth
 	var launchArgs []string