fix: ShowBrowser merge behavior and consistent browser defaults

- Change ShowBrowser from bool to *bool so nil means "don't override"
  in mergeOptions(), fixing the bug where it always overwrote the base
- Add Bool() helper for convenient *bool construction
- Align NewInteractiveBrowser default from Chromium to Firefox to match
  NewBrowser
- Update README example and CLI flags for the *bool change

Closes #15, #16

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

cmd/browser/pkg/browser/flags.go

@@ -70,7 +70,9 @@ func FromCommand(ctx context.Context, cmd *cli.Command) (extractor.Browser, erro
 		opts.CookieJar = cookies
 	}
 
-	opts.ShowBrowser = cmd.Bool("visible")
+	if cmd.IsSet("visible") {
+		opts.ShowBrowser = extractor.Bool(cmd.Bool("visible"))
+	}
 
 	return extractor.NewBrowser(ctx, opts)
 }