fix: enable selection-to-conversation popup in the markdown editor

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.

src/client/TextEditor.tsx

@@ -141,11 +141,11 @@ export function TextEditor(props: FileViewerProps) {
           ...defaultKeymap,
           ...historyKeymap,
         ]),
-        // Selection popup (the catch-all code viewer only): a non-empty
+        // Selection popup (the code and markdown editors): a non-empty
         // selection anchors the floating "add to conversation" button above
         // its head. Scrolling (geometry/viewport change) or losing focus
         // hides it; typing collapses the selection and hides it too.
-        ...(viewerId === 'code' ? [
+        ...(viewerId === 'code' || viewerId === 'markdown' ? [
           CodeMirrorView.updateListener.of((update) => {
             if (update.geometryChanged || update.viewportChanged) {
               hidePopup()