fix(flowchart): prevent preview-source duplication; v4.9.3

The flowchart preview pane accumulated raw Mermaid source when the user
fired several addNode mutations within the 250ms debounce window —
mermaid.run({nodes:[div]}) is async, so the previous render's
<div class="mermaid"> (still carrying the source text) sat in
.flowchart-preview-render when the next render cleared the target. The
first render's eventual element.innerHTML=svg landed on a detached node,
but the visible preview pane had a stack of stale <div class="mermaid">
elements carrying the source.

Fixed by switching renderFlowChartMermaid (src/renderer.js) to
replaceChildren() and adding a per-target WeakSet in-flight tracker so
the new render always starts from a clean slate and the previous render's
eventual innerHTML=svg is harmless on a detached node.

Added a second regression test that fires 7 mutations inside the debounce
with a renderMermaid mock that mimics mermaid.run's async innerHTML=svg
closure.

Amit Haridas

package.json

@@ -1,6 +1,6 @@
 {
   "name": "markdown-converter",
-  "version": "4.9.2",
+  "version": "4.9.3",
   "description": "Professional Markdown editor and universal file converter with PDF editing, batch processing, and syntax highlighting",
   "main": "src/main.js",
   "scripts": {