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three different UA strings existed (two at 142.0, one outdated at 133.0).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>

cmd/browser/pkg/browser/flags.go

@@ -16,7 +16,7 @@ var Flags = BrowserFlags{
 		Name:        "user-agent",
 		Aliases:     []string{"ua"},
 		Usage:       "User-Agent to use for requests",
-		DefaultText: "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:133.0) Gecko/20100101 Firefox/133.0",
+		DefaultText: extractor.DefaultUserAgent,
 	},
 	&cli.StringFlag{
 		Name:        "timeout",

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{

playwright.go

@@ -36,6 +36,9 @@ const (
 	BrowserWebKit   BrowserSelection = "webkit"
 )
 
+// DefaultUserAgent is the user-agent string used by all browser instances.
+const DefaultUserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:147.0) Gecko/20100101 Firefox/147.0"
+
 // Bool returns a pointer to the given bool value.
 func Bool(v bool) *bool { return &v }
 
@@ -44,7 +47,7 @@ type Size struct {
 	Height int
 }
 type BrowserOptions struct {
-	UserAgent string                     // If empty, defaults to "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:133.0) Gecko/20100101 Firefox/133.0"
+	UserAgent string                     // If empty, defaults to DefaultUserAgent
 	Browser   BrowserSelection // If unset defaults to Firefox.
 	Timeout   *time.Duration             // If unset defaults to 30 seconds timeout.  If set to 0, no timeout
 
@@ -94,7 +97,7 @@ func playwrightCookieToCookie(cookie playwright.Cookie) Cookie {
 func NewBrowser(ctx context.Context, opts ...BrowserOptions) (Browser, 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,
 	}, opts)