fix: resolve TipTap v3 type errors in RichEditor

- Cast editor.storage to any for tiptap-markdown's getMarkdown (untyped extension storage)
- Replace setContent(value, false) with setContent(value, { emitUpdate: false }) for TipTap v3 API

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ui/rich-editor.tsx

@@ -28,7 +28,8 @@ export function RichEditor({ value, onChange, placeholder, className }: Props) {
     content: value,
     immediatelyRender: false,
     onUpdate({ editor: e }) {
-      const md = e.storage.markdown?.getMarkdown?.() ?? e.getText();
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const md = (e.storage as any).markdown?.getMarkdown?.() ?? e.getText();
       onChange(md);
     },
   });
@@ -36,9 +37,10 @@ export function RichEditor({ value, onChange, placeholder, className }: Props) {
   // Sync external value changes (e.g. when a new lesson is loaded).
   useEffect(() => {
     if (!editor) return;
-    const current = editor.storage.markdown?.getMarkdown?.() ?? editor.getText();
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const current = (editor.storage as any).markdown?.getMarkdown?.() ?? editor.getText();
     if (current !== value) {
-      editor.commands.setContent(value, false);
+      editor.commands.setContent(value, { emitUpdate: false });
     }
   }, [value, editor]);