refactor: Remove AlchemyHTMLElement base class

No component extends it anymore after moving every custom element to
plain HTMLElement and connectedCallback. The base class encouraged the
very constructor-time DOM work that violated the Web Components spec
and broke cloneNode (SortableJS drag and drop). Dropping it removes
the tempt to repeat that pattern and the documentation now points new
components at extending HTMLElement directly.

Author: tvdeyen

AGENTS.md

@@ -188,7 +188,7 @@ The admin interface uses a **hybrid architecture**:
 
 **Modern Web Components** (preferred for new features):
 - Custom elements in `app/javascript/alchemy_admin/components/`
-- Base class: `AlchemyHTMLElement` (extends `HTMLElement`)
+- Extend `HTMLElement` directly; do all DOM work in `connectedCallback`
 - Examples: `alchemy-sitemap`, `alchemy-element-editor`, `alchemy-datepicker`
 - Vanilla JavaScript (no framework dependency)