feat: real-time gate messages via SSE + faster push/pull intervals

- Add Server-Sent Events endpoint at GET /api/mesh/gate/stream that
  broadcasts ALL gate events to connected frontends (privacy: no
  per-gate subscriptions, clients filter locally)
- Hook SSE broadcast into all gate event entry points: local append,
  peer push receiver, and pull loop
- Reduce push/pull intervals from 30s to 10s for faster relay sync
- Add useGateSSE hook for frontend EventSource integration
- GateView + MeshChat use SSE for instant refresh, polling demoted
  to 30s fallback

Latency: same-node instant, cross-node ~10s avg (was ~34s)

frontend/src/hooks/useGateSSE.ts

@@ -0,0 +1,33 @@
+import { useEffect, useRef } from 'react';
+import { API_BASE } from '@/lib/api';
+
+/**
+ * Subscribe to the backend SSE gate-event stream.
+ * Delivers ALL gate events (encrypted blobs) — the client filters by gate_id locally.
+ * The server never learns which gates a client cares about (privacy-preserving broadcast).
+ *
+ * Falls back gracefully: if the stream fails the browser's EventSource auto-reconnects.
+ */
+export function useGateSSE(onEvent: (gateId: string) => void) {
+  const callbackRef = useRef(onEvent);
+  callbackRef.current = onEvent;
+
+  useEffect(() => {
+    const es = new EventSource(`${API_BASE}/api/mesh/gate/stream`);
+
+    es.onmessage = (e) => {
+      try {
+        const data = JSON.parse(e.data);
+        if (data.gate_id && typeof data.gate_id === 'string') {
+          callbackRef.current(data.gate_id);
+        }
+      } catch {
+        /* ignore parse errors */
+      }
+    };
+
+    // Browser auto-reconnects EventSource on error — no manual retry needed.
+
+    return () => es.close();
+  }, []);
+}