fix(mdviewer): use morphdom for incremental DOM updates to prevent scroll jump

Replace full innerHTML replacement with morphdom diffing so unchanged
elements (images, code blocks) stay in the DOM during CM→preview sync.
Prevents image reloads and layout shifts when editing text.

Author: abose

src-mdviewer/package-lock.json

@@ -20,6 +20,7 @@
         "medium-zoom": "^1.1.0",
         "mermaid": "^11.12.2",
         "mitt": "^3.0.1",
+        "morphdom": "^2.7.8",
         "prismjs": "^1.29.0",
         "turndown": "^7.2.0",
         "turndown-plugin-gfm": "^1.0.2"

src-mdviewer/package.json

@@ -4,6 +4,7 @@ Prism.manual = true;
 import "../core/prism-languages.js";
 import "../styles/syntax/prism-light.css";
 import "../styles/syntax/prism-dark.css";
+import morphdom from "morphdom";
 import { on } from "../core/events.js";
 import { getState } from "../core/state.js";
 import { t } from "../core/i18n.js";
@@ -56,7 +57,9 @@ export function initViewer() {
         const content = getContentEl();
         if (!content) return;
 
-        content.innerHTML = parseResult.html;
+        const newContent = document.createElement("div");
+        newContent.innerHTML = parseResult.html;
+        morphdom(content, newContent, { childrenOnly: true });
         content.dir = "auto";
 
         wrapTables();