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three different UA strings existed (two at 142.0, one outdated at 133.0).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