feat: relay-grouped participant rendering + relay_label in protocol

RoomParticipant now has optional relay_label field. Desktop client
groups participants by relay: "This Relay" (green dot) for local,
peer label (blue dot) for federated. Shows all relays in the chain
including intermediate ones.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src/style.css

@@ -441,6 +441,35 @@ button.primary:disabled { opacity: 0.5; cursor: not-allowed; }
   border-radius: 8px;
 }
 
+/* ── Relay group headers ── */
+.relay-group-header {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  font-size: 11px;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  color: var(--text-dim);
+  padding: 6px 0 2px;
+  border-top: 1px solid #ffffff08;
+  margin-top: 4px;
+}
+
+.relay-group-header:first-child {
+  border-top: none;
+  margin-top: 0;
+}
+
+.relay-dot-small {
+  width: 6px;
+  height: 6px;
+  border-radius: 50%;
+  display: inline-block;
+}
+
+.relay-dot-small.green { background: var(--green); }
+.relay-dot-small.blue { background: #60a5fa; }
+
 /* ── Controls ── */
 .controls {
   display: flex;