feat: in-room text chat over the gossip plane

Add room text chat riding the existing iroh-gossip topic (same layer as the
presence roster). New GossipMessage::Chat { name, text, ts }; the gossip loop
forwards it as RoomEvent::ChatMessage, core relays it to the UI as
UiEvent::ChatMessage, and RoomState::send_chat broadcasts an authored line
(display name from self-state, ms timestamp). CoreCommand::SendChat sends; our
own author is suppressed by the existing self-echo guard, so the UI echoes our
sent line locally instead.

UI: a full-width chat dock along the bottom of the room (the chosen layout) —
bottom-anchored scrollback with per-sender name colouring (green = you), an
input with Enter-to-send + a Send button, history capped at 300 lines. The room
window default grows to 900x760 so the dock doesn't squeeze the controls column.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/messages.rs

@@ -29,6 +29,8 @@ pub enum CoreCommand {
     /// Start/stop recording the call to a local WAV (your mic + the incoming
     /// mix). No-op start if already recording / not in a call.
     SetRecording(bool),
+    /// Broadcast a room text-chat message. No-op when not in a call.
+    SendChat(String),
 }
 
 #[derive(Debug, Clone)]
@@ -51,5 +53,8 @@ pub enum UiEvent {
     RecordingStarted { path: String },
     /// Call recording stopped; carries the finished WAV path.
     RecordingStopped { path: String },
+    /// A room text-chat message arrived from a peer (never our own — local
+    /// messages are echoed by the UI on send).
+    ChatMessage { name: String, text: String },
     Error(String),
 }