Annotate session state structs with allow(dead_code) for future use

The ArchiveSession and RestoreSession structs in the handler state machine
contain fields (session_id, file_count, compression, bytes_received,
bytes_sent) that are stored for future use in resume-after-interruption
logic and detailed transfer statistics. Currently they are only written
during state transitions but not yet read back. Add #[allow(dead_code)]
annotations to suppress compiler warnings while keeping the fields
available for the planned transfer progress tracking and interrupted
archive resume features.