fix: harden persistence checks and export timeout

lib/session-storage.ts

@@ -75,6 +75,18 @@ export function isIndexedDBAvailable(): boolean {
     }
 }
 
+// Check if IndexedDB is actually usable (not just present).
+export async function isIndexedDBUsable(): Promise<boolean> {
+    if (!isIndexedDBAvailable()) return false
+    try {
+        const db = await getDB()
+        db.close()
+        return true
+    } catch {
+        return false
+    }
+}
+
 // CRUD Operations
 export async function getAllSessionMetadata(): Promise<SessionMetadata[]> {
     if (!isIndexedDBAvailable()) return []