Implement profile deletion with undo functionality

- Add delete_profile() to profiles.rs - moves profile to undo stack
- Add undo_history field and undo_delete() method to App struct
- Add cli_delete_profile() and cli_undo_delete() CLI commands
- Add delete-profile <npub> and undo-delete to USAGE
- Update CHECKPOINT-encryption.md checkpoint

Verification: cargo test (91 passed), clippy clean, fmt clean, release build successful

concept3.svg

@@ -0,0 +1,11 @@
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