feat(ui): reconcile backup UI changes with current routing and interaction features

ui/src/components/MarkdownEditor.tsx

@@ -10,9 +10,11 @@ import {
   type DragEvent,
 } from "react";
 import {
+  CodeMirrorEditor,
   MDXEditor,
   codeBlockPlugin,
   codeMirrorPlugin,
+  type CodeBlockEditorDescriptor,
   type MDXEditorMethods,
   headingsPlugin,
   imagePlugin,
@@ -90,6 +92,14 @@ const CODE_BLOCK_LANGUAGES: Record<string, string> = {
   yml: "YAML",
 };
 
+const FALLBACK_CODE_BLOCK_DESCRIPTOR: CodeBlockEditorDescriptor = {
+  // Keep this lower than codeMirrorPlugin's descriptor priority so known languages
+  // still use the standard matching path; this catches malformed/unknown fences.
+  priority: 0,
+  match: () => true,
+  Editor: CodeMirrorEditor,
+};
+
 function detectMention(container: HTMLElement): MentionState | null {
   const sel = window.getSelection();
   if (!sel || sel.rangeCount === 0 || !sel.isCollapsed) return null;
@@ -247,7 +257,10 @@ export const MarkdownEditor = forwardRef<MarkdownEditorRef, MarkdownEditorProps>
       linkPlugin(),
       linkDialogPlugin(),
       thematicBreakPlugin(),
-      codeBlockPlugin(),
+      codeBlockPlugin({
+        defaultCodeBlockLanguage: "txt",
+        codeBlockEditorDescriptors: [FALLBACK_CODE_BLOCK_DESCRIPTOR],
+      }),
       codeMirrorPlugin({ codeBlockLanguages: CODE_BLOCK_LANGUAGES }),
       markdownShortcutPlugin(),
     ];