F2.4 Messaging: conversations with ack, retry and reply-ack #192

Spec: to write first (docs/features/F2.4-….md), from the scope row.

A Messages panel with a conversation per correspondent, on APRS.NET's messaging engine:

• send with message IDs, retries and reply-acks; each message shows queued, sent, acked, rejected or given up;
• received messages acked and delivered once; unread counts;
• retry profiles (default, APRS-IS, none, slow HF modem); an offline queue that sends when the connection is back;
• gated by F2.1's Transmit switch and, on a gateway, F2.9's operator.

Gap: G10. Blocked by APRS-NET/APRS.NET#189 (sub-issues #191, #192).