- Save the latest diagram of every session 1 second after each change
(AI write, browser edit, history restore) to ~/.next-ai-drawio/<id>.drawio,
keep the newest 50, flush on shutdown; DRAWIO_DATA_DIR changes the folder
and "off" disables it, like the web app's IndexedDB sessions
- start_session names the file, so a resumed conversation can reopen the
diagram with load_diagram after the MCP process restarted
- Fix PNG/SVG exports randomly timing out: a previous export's 10 second
timer cleared the export in progress, and a late reply could be taken
for the current one; exports are now numbered
