mavweb: add notifications history page at /notifications

RecentNudges IPC method, store adapter, dispatch, and client proxy.
Web UI at /notifications showing recent nudge history with color-coded
outcomes, nav links from /dash and /history.

cmd/mavweb/dash.html

@@ -10,7 +10,7 @@
  .updated{color:#666;font-size:.85rem;margin-top:-.5rem;margin-bottom:1rem}
  nav a{color:#8cf;margin-right:1rem;font-size:.85rem}
 </style>
-<nav><a href=/history>history</a> <a href=/trace>trace</a></nav>
+<nav><a href=/history>history</a> <a href=/trace>trace</a> <a href=/notifications>notifications</a></nav>
 <h2>presence</h2>
 <p><span class={{.Presence.Bucket}}>{{.Presence.Bucket}}</span> — score {{printf "%.2f" .Presence.Score}} ({{ago .Presence.Updated}})</p>
 <div class=updated>обновляется каждые 10с</div>