The "add to conversation" selection popup was registered only for the
catch-all code viewer (viewerId === 'code'), so selecting text in the
CodeMirror editor of a markdown file produced no popup; insertion only
worked in markdown preview mode via the separate DOM-selection handler.
Extend the CodeMirror selection popup to the markdown editor
(viewerId === 'markdown') so line-exact selections can be injected into
the composer draft (\`\`\`rel/path:start-end\`\`\` payload), matching
preview-mode behavior and the code viewer.
