Fix/226 autosave pen lag

[devmobasa]

Fixes #226

Summary

Fixes the first-stage failure modes behind #226 for image-heavy sessions:

• defer autosave while pointer/stylus, board pan, zoom pan, toolbar drag, or move interactions are active
• avoid autosave due-timeout spinning by scheduling a short retry window
• bound autosave history fallback work so periodic saves do not scan deeply on the UI loop
• preserve oversized sessions with recovery artifacts when normal save exceeds the configured cap
• load recovery artifacts safely, while falling back to normal sessions if recovery is corrupt/ empty/too large
• preflight page/board duplication and cross-board page copy so clone-heavy image sessions are blocked before becoming unsaveable

Details

This PR adds a recovery safety net for oversized saves and prevents common duplicate/copy actions from creating sessions that the normal save path cannot persist.