fix(electron): prevent beforeunload prompt by using autosave

- Enable draw.io autosave and handle autosave events to update chartXML
- Clear modified state after autosave to avoid beforeunload prompts
- Disable confirmExit in draw.io configuration
- Set modified=false and keepmodified=false URL parameters
- Fix session save condition to also save when only diagram exists
- Fix: Do not close shared IndexedDB connection in isIndexedDBUsable()

lib/session-storage.ts

@@ -75,6 +75,19 @@ export function isIndexedDBAvailable(): boolean {
     }
 }
 
+// Check if IndexedDB is actually usable (not just present).
+// Note: Do NOT close the db here - getDB() returns a shared singleton connection
+// that other code depends on.
+export async function isIndexedDBUsable(): Promise<boolean> {
+    if (!isIndexedDBAvailable()) return false
+    try {
+        await getDB()
+        return true
+    } catch {
+        return false
+    }
+}
+
 // CRUD Operations
 export async function getAllSessionMetadata(): Promise<SessionMetadata[]> {
     if (!isIndexedDBAvailable()) return []