fix: wait for canPersist check before rendering DrawIoEmbed

Fixes diagram not restoring after refresh. The issue was:
1. DrawIoEmbed rendered with canPersist=false
2. Session restore loaded diagram into iframe
3. canPersist changed to true, causing iframe remount
4. New iframe loaded empty, losing the diagram

Now we wait for isIndexedDBUsable() to complete before rendering,
avoiding the remount entirely.

app/[lang]/page.tsx

@@ -33,6 +33,7 @@ export default function Home() {
     const [isDrawioReady, setIsDrawioReady] = useState(false)
     const [isElectron, setIsElectron] = useState(false)
     const [canPersist, setCanPersist] = useState(false)
+    const [canPersistChecked, setCanPersistChecked] = useState(false)
     const [drawioBaseUrl, setDrawioBaseUrl] = useState(
         process.env.NEXT_PUBLIC_DRAWIO_BASE_URL || "https://embed.diagrams.net",
     )
@@ -86,6 +87,7 @@ export default function Home() {
         void (async () => {
             const usable = await isIndexedDBUsable()
             setCanPersist(usable)
+            setCanPersistChecked(true)
         })()
         setIsLoaded(true)
     }, [pathname, router])
@@ -189,12 +191,12 @@ export default function Home() {
                         }`}
                     >
                         <div className="h-full rounded-xl overflow-hidden shadow-soft-lg border border-border/30 relative">
-                            {isLoaded && (
+                            {isLoaded && canPersistChecked && (
                                 <div
                                     className={`h-full w-full ${isDrawioReady ? "" : "invisible absolute inset-0"}`}
                                 >
                                     <DrawIoEmbed
-                                        key={`${drawioUi}-${darkMode}-${currentLang}-${isElectron}-${canPersist}`}
+                                        key={`${drawioUi}-${darkMode}-${currentLang}-${isElectron}`}
                                         ref={drawioRef}
                                         autosave
                                         onAutoSave={handleDrawioAutoSave}