feat(workspace): numbered lists and reliable markdown paste

- block type 'numbered' (/number): 1., a., i. by nesting, continues on
  Enter, an empty item ends the list; markdown export/import as 1.
- pasting: parser moved to lib/pasteOutline with tests; blank lines split
  paragraphs, numbered lists keep their type, indentation levels follow
  the widths in use (2, 3 or 4 spaces, tabs)
- text that stays in the focused block after pasting (a paragraph with
  line breaks) was dropped; the block now takes it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/DocumentEditor.tsx

@@ -746,6 +746,8 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
         lines.push(`${indent}- \`\`\`${b.lang || ''}\n${b.content}\n${indent}  \`\`\``);
       } else if (b.type === 'divider') {
         lines.push(`${indent}- ---`);
+      } else if (b.type === 'numbered') {
+        lines.push(`${indent}1. ${b.content}`);
       } else {
         lines.push(`${indent}- ${b.content}`);
       }