Fix image-heavy session persistence limits and warnings

[devmobasa]

Summary

• check session size limits against the final bytes written after compression
• add image paste preflight so oversized pasted images are blocked before making unsavable session state
• preserve fitting undo history when possible, including depth 1 near the limit
• add near-limit, history-trimmed, visible-only, and save-failure user warnings
• harden load/save behavior around expanded compressed payload size
• improve session save, clipboard image, and shutdown diagnostics

Notes

Existing users with an explicit session.max_file_size_mb = 10 keep that value. New configs or missing values use the new 50 MiB default.