fix: restore diagram when chartXML changes after iframe ready

The previous logic only restored on iframe ready, but chartXML might
be set AFTER the iframe is ready (session loaded after iframe).

Now we track the last restored XML and load whenever chartXML changes
to a new real diagram while iframe is ready.

contexts/diagram-context.tsx

@@ -57,8 +57,6 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
     const pngResolverRef = useRef<((value: string) => void) | null>(null)
     // Track if we're expecting an export for history (user-initiated)
     const expectHistoryExportRef = useRef<boolean>(false)
-    // Track if diagram has been restored after DrawIO remount (e.g., theme change)
-    const hasDiagramRestoredRef = useRef<boolean>(false)
     // Track latest chartXML for restoration after remount
     const chartXMLRef = useRef<string>("")
 
@@ -80,22 +78,19 @@ export function DiagramProvider({ children }: { children: React.ReactNode }) {
     }, [chartXML])
 
     // Restore diagram when DrawIO becomes ready after remount (e.g., theme/UI change)
+    // Also restore when chartXML changes while DrawIO is ready (e.g., session loaded after iframe ready)
+    const lastRestoredXmlRef = useRef<string>("")
     useEffect(() => {
-        // Reset restore flag when DrawIO is not ready (preparing for next restore cycle)
-        if (!isDrawioReady) {
-            hasDiagramRestoredRef.current = false
-            return
+        if (!isDrawioReady || !drawioRef.current) return
+        // Only load if we have a real diagram and it's different from what we already loaded
+        if (
+            isRealDiagram(chartXML) &&
+            chartXML !== lastRestoredXmlRef.current
+        ) {
+            lastRestoredXmlRef.current = chartXML
+            drawioRef.current.load({ xml: chartXML })
         }
-        // Only restore once per ready cycle
-        if (hasDiagramRestoredRef.current) return
-        hasDiagramRestoredRef.current = true
-
-        // Restore diagram from ref if we have one
-        const xmlToRestore = chartXMLRef.current
-        if (isRealDiagram(xmlToRestore) && drawioRef.current) {
-            drawioRef.current.load({ xml: xmlToRestore })
-        }
-    }, [isDrawioReady])
+    }, [isDrawioReady, chartXML])
 
     // Track if we're expecting an export for file save (stores raw export data)
     const saveResolverRef = useRef<{