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>

interactive.go

@@ -60,7 +60,7 @@ func NewInteractiveBrowser(ctx context.Context, opts ...BrowserOptions) (Interac
 	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",
-		Browser:   BrowserChromium,
+		Browser:   BrowserFirefox,
 		Timeout:   &thirtySeconds,
 		Dimensions: Size{
 			Width:  1280,