* 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: persist diagram-only saves and ref typing
* fix: harden persistence checks and export timeout