refactor(markdown-editor): type editor.storage.markdown via module augmentation

Adds a tiptap-markdown.d.ts that augments @tiptap/core's Storage interface,
removing the two `(editor.storage as any).markdown.getMarkdown()` casts.
The augmentation is internal to the build (not referenced from index.d.ts)
so it does not leak to consumers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hendrikebbers

src/components/markdown-editor.tsx

@@ -115,8 +115,7 @@ export function MarkdownEditor({ value, onChange, placeholder }: MarkdownEditorP
     content: value,
     immediatelyRender: false,
     onUpdate: ({ editor: ed }) => {
-      // eslint-disable-next-line @typescript-eslint/no-explicit-any
-      onChange((ed.storage as any).markdown.getMarkdown());
+      onChange(ed.storage.markdown.getMarkdown());
     },
     editorProps: {
       attributes: {
@@ -127,8 +126,7 @@ export function MarkdownEditor({ value, onChange, placeholder }: MarkdownEditorP
 
   useEffect(() => {
     if (!editor) return;
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    const currentMarkdown = (editor.storage as any).markdown.getMarkdown();
+    const currentMarkdown = editor.storage.markdown.getMarkdown();
     if (value !== currentMarkdown) {
       editor.commands.setContent(value);
     }

src/types/tiptap-markdown.d.ts

@@ -0,0 +1,11 @@
+import type { MarkdownStorage } from "tiptap-markdown";
+
+// Augment tiptap's Storage interface so editor.storage.markdown is typed.
+// See https://tiptap.dev/docs/editor/extensions/custom-extensions/extend-existing#storage
+declare module "@tiptap/core" {
+  interface Storage {
+    markdown: MarkdownStorage;
+  }
+}
+
+export {};