feat: keep incoming deleted messages visible locally

[rhcp011235]

Summary

• Messages deleted by other users no longer disappear from the local message list
• Only outgoing messages (sent by the local user) are removed when a delete event is received
• Incoming messages from other users survive deletion updates and remain visible

Implementation

Modified processMessagesDeleted() and processNonChannelMessagesDeleted() in data_session.cpp to skip destroy() for any message where item->out() returns false.

Test plan

• Have another user delete a message in a shared chat — confirm it stays visible on your end
• Delete one of your own messages — confirm it still disappears normally
• Test in both regular chats and channels

🤖 Generated with Claude Code

[rhcp011235]

confirmed this to work as well

[Burhanverse]

The PR only demonstrates the concept but would need substantially more work to be a proper feature:

• A settings toggle in FAgram settings
• Visual indication of deleted messages
• Proper handling of the historiesToCheck logic that gets skipped
• Ideally, local persistence so anti-deleted messages survive restarts

I would not merge this PR as-is. It's more of a proof-of-concept than a complete feature implementation.