[gx1400]

UI for F2.4 (#192), stacked on #229.

• Messages panel, a tab beside Details:
  • Message a callsign (Start, or Enter).
  • Conversations, newest first, with unread counts.
  • The open conversation: theirs on the left, ours on the right with the accent outline. Each message shows its time and state in words (Waiting to send, Sent, Sent again try 2 of 6, Delivered, Rejected by the station, Not delivered, Not acknowledged: transmitting is off), with Cancel and Send again.
  • Send via in an expander whose header says where the next message goes: Automatic or ticked sources, plus Retries.
  • The compose box counts how many messages the text makes (2 messages · 100 of 300), and Enter sends.
• Send message in a station's details opens its conversation.
• Status bar: "1 new message" appears while any are unread and opens the panel.
• Gateway browsers: the panel says messages are the operator's until sign-in.
• Bug fixed: saving Send via no longer resets the ticks. The engine now also lists conversations that have settings but no messages yet.

Tests

• New: MessagesTests (6). The layout tests now include the new panel.
• Results: Tiles 17, Gateway 110, Core 325, UI 477. All pass, and csharpier is clean.

Closes #192