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>

playwright.go

@@ -36,6 +36,9 @@ const (
 	BrowserWebKit   BrowserSelection = "webkit"
 )
 
+// Bool returns a pointer to the given bool value.
+func Bool(v bool) *bool { return &v }
+
 type Size struct {
 	Width  int
 	Height int
@@ -49,7 +52,7 @@ type BrowserOptions struct {
 	// browser into the cookie jar for each request.
 	CookieJar
 
-	ShowBrowser bool // If false, browser will be headless
+	ShowBrowser *bool // If nil, defaults to false (headless). Set to ptr to override.
 
 	Dimensions Size
 	DarkMode   bool