Summary

Add horizontal rule (separator) support to the rich text editor, allowing
users to insert <hr> elements by typing --- in markdown style.

Changes

• Add TipTap HorizontalRule extension to the rich text editor formatting extensions
• Add horizontalRule rendering in renderBlock with styleable classes via text styles
• Add horizontalRule to DEFAULT_TEXT_STYLES with Separator label and default border styling
• Add horizontalRule to sublayer icon/name maps so it appears as "Separator" in the layers tree
• Add contentBlockToStyleKey mapping for horizontalRule so design changes target the HR element
• Show BorderControls in the right sidebar when a Separator sublayer is selected
• Add horizontalRule to contentHasBlockElements and extractInlineNodesFromRichText in tiptap-utils
• Add editor CSS for <hr> inside rich text
• Update standalone Separator layer template to use border-t-[1px] border-[#aeaeae]

Test plan

• In a rich text field, type --- on an empty line and press Enter - a horizontal rule appears
• The separator shows in the layers tree as "Separator" with the separator icon
• Click the Separator sublayer in the layers tree - it gets selected and BorderControls appear in the right sidebar
• Adjust border settings on the selected Separator - changes apply to the HR element, not the parent rich text
• Publish the page - the <hr> renders with correct classes in the published output
• Add a standalone Separator element from the Element Library - uses the updated border styling