fix(2.x): fix forum widget layout and admin editor drag-and-drop

[imorland]

Summary

• Replace broken vnode-mutation approach with proper PageStructure extension points: top/bottom sections via mainItems() (wrapped in .container for correct width), end section via containerItems() as a flex sibling of the content column
• Guard all widget sections against rendering when empty (previously relied on CSS :empty which left orphan container divs)
• Fix admin editor DOMException: Node.removeChild crash when dragging widgets — add onbeforeupdate={() => false} to prevent Mithril re-rendering the Sortable.js-owned DOM, and remove the cleanupLayout workaround that existed to undo those re-renders
• Fix stable key on layout widget list items (was random, causing Mithril key mismatches)
• Update README import paths to use ext: prefix per Flarum 2.0 module import convention