Refactor and simplify plugin to reduce code size and improve structure

[lex127]

Goal

The plugin currently has about 2,000 lines of code. A lot of it is hard to follow and change safely:

• webspellchecker.php is a 798-line class with 18+ mixed responsibilities (bootstrap, context detection, enqueue, AJAX, content cleanup, config builder, etc.).
• should_enable_in_context() is 116 lines with 41 conditionals and the same arrays of screen IDs repeated three times.
• A 620-line vendored Settings API (WeDevs) ships inside vendor/ and contains a deprecated create_function() call — see Fix deprecated create_function() call (fatal on PHP 8.0+) #30.
• Service endpoints (svc.webspellchecker.net, spellcheck31/api, the bundle URL) are hard-coded in five places across PHP and JS. On-premises customers cannot point the plugin at a private WSC instance without editing the source.
• Content cleanup runs a regex over HTML with a length-ratio safety check — fragile.
• Four JavaScript files with duplicated constants and no module structure.

Plan

• 6–8 small classes with a single responsibility each.

Non-goals

• No new features.
• No PHP version bump (stays 7.4).
• No Composer, no React admin page, no build step for PHP.
• No breaking changes to the wsc_proofreader option or public filter names.

Related

• Fix deprecated create_function() call (fatal on PHP 8.0+) #30 — PHP 8 fatal fix (subset of the vendor removal planned here; ship that one first as a patch).