fix: consolidate user-agent strings into DefaultUserAgent constant

Define DefaultUserAgent (Firefox/147.0) in playwright.go and reference
it from NewBrowser, NewInteractiveBrowser, and CLI flags. Previously
three different UA strings existed (two at 142.0, one outdated at 133.0).

Closes #17

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

interactive.go

@@ -59,7 +59,7 @@ type interactiveBrowser struct {
 func NewInteractiveBrowser(ctx context.Context, opts ...BrowserOptions) (InteractiveBrowser, error) {
 	var thirtySeconds = 30 * time.Second
 	opt := mergeOptions(BrowserOptions{
-		UserAgent: "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:142.0) Gecko/20100101 Firefox/142.0",
+		UserAgent: DefaultUserAgent,
 		Browser:   BrowserFirefox,
 		Timeout:   &thirtySeconds,
 		Dimensions: Size{