Don't apply stale message edits

This is a follow-up to discussion in https://github.com/chatmail/core/issues/6806#issuecomment-3005608761

Steps to reproduce the bug:
- Send a message from device A and receive it on device B
- Disconnect device A from network
- Edit the message on both devices in different ways
- Connect device A to network and see that the devices have exchanged their edits

The expected behavior is that the last edit wins on both devices. A suggested fix is to rename `Param::IsEdited` to `EditTimestamp` and store the edit "sent" timestamp there instead of 0/1. This change is backward-compatible. If only one device is updated to the fixed version, it will reject stale edits, but devices running old versions will still apply all edits, this is not worse than the current behavior.