feat(wordpress): settings page with PDF links report, page scoping and account check

[bendersej]

Background

The WordPress plugin's settings page showed a single text field. Admins could not tell which PDFs the plugin takes over, could not try it on one page before going live, and did not know where filled PDFs go.

Changes

• WordPress plugin settings page, rebuilt with native WP admin styling:
  • PDFs on your site: every page and post whose content links a PDF, with where each link opens (SimplePDF, browser, or an error) and why, plus a Get started state when there are none
  • Where it runs: everywhere (default, unchanged behaviour) or only on picked pages and posts, drafts included, to test before going live
  • Account card: what an account adds, or the linked account with a check that it exists
  • Guides: links to the help center
• One-time review notice on the settings screen once a linked account exists and a PDF link opens in SimplePDF; dismissible for good (nonce-checked AJAX)
• uninstall.php removes every option the plugin stores
• The plugin keeps the published @simplepdf/web-embed-pdf 1.8.4 bundle; its version is now one constant used for cache busting. Verified in WordPress Playground, and php -l is clean on PHP 5.6.

Notes

• Screenshots and recordings are on the internal PR (SimplePDF/simple-pdf#415).
• Blocked on the next @simplepdf/web-embed-pdf release (rebuilt on @simplepdf/embed, with PDF links detected in any extension case, query or fragment): the plugin pins it before it is published.