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>

README.md

@@ -118,7 +118,7 @@ extractor.BrowserOptions{
     Browser:       extractor.BrowserFirefox, // or BrowserChromium, BrowserWebKit
     Timeout:       &timeout,                 // default 30s, 0 for no timeout
     CookieJar:     jar,                      // load/save cookies automatically
-    ShowBrowser:   true,                     // show browser window (non-headless)
+    ShowBrowser:   extractor.Bool(true),     // show browser window (non-headless)
     Dimensions:    extractor.Size{1280, 720},
     DarkMode:      true,
     ServerAddress: "ws://...",               // remote Playwright server