fix: preserve multi-page diagrams after export (#895)

* fix: preserve multi-page diagrams after export

* fix: keep chartXML sourced from autosave to preserve multi-page state

The export event's data.xml (xmlsvg format) contains compressed <diagram>
payloads, which would break applyDiagramOperations/replaceNodes consumers
that need plain <root> elements. Instead of writing export results into
chartXML, stop overwriting it entirely: autosave already delivers the full
uncompressed multi-page document, and loadDiagram covers AI-driven updates.

Also stop overwriting chartXMLRef with the page-only export before sending
a chat message, so session persistence never sees single-page XML.

Keep the data.xml preference for .drawio file downloads (compressed pages
are a valid drawio format).

---------

Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

components/chat-panel.tsx

@@ -830,10 +830,6 @@ export default function ChatPanel({
                 let chartXml = await onFetchChart()
                 chartXml = formatXML(chartXml)
 
-                // Update ref directly to avoid race condition with React's async state update
-                // This ensures edit_diagram has the correct XML before AI responds
-                chartXMLRef.current = chartXml
-
                 // Build user text by concatenating input with pre-extracted text
                 // (Backend only reads first text part, so we must combine them)
                 const parts: any[] = []