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>

browser_init.go

@@ -70,8 +70,9 @@ func initBrowser(opt BrowserOptions) (*browserInitResult, error) {
 	}
 
 	if launch {
+		headless := opt.ShowBrowser == nil || !*opt.ShowBrowser
 		browser, err = bt.Launch(playwright.BrowserTypeLaunchOptions{
-			Headless: playwright.Bool(!opt.ShowBrowser),
+			Headless: playwright.Bool(headless),
 		})
 		if err != nil {
 			return nil, fmt.Errorf("failed to launch browser: %w", err)
@@ -154,7 +155,9 @@ func mergeOptions(base BrowserOptions, opts []BrowserOptions) BrowserOptions {
 		if o.UseLocalOnly {
 			base.UseLocalOnly = true
 		}
-		base.ShowBrowser = o.ShowBrowser
+		if o.ShowBrowser != nil {
+			base.ShowBrowser = o.ShowBrowser
+		}
 	}
 	return base
 }