mavweb: add /history page for command history

- New /history route displays recent facts in a dedicated page with
  voided-row styling (line-through + opacity + badge).
- Navigation link added to /dash page header.
- Handler calls core.RecentFacts(ctx, 200) and renders historyTmpl.

cmd/mavweb/dash.html

@@ -8,7 +8,9 @@
  .pending{color:#fc6}.acted{color:#6c6}.ignored{color:#888}.snoozed{color:#c9f}
  .pres{color:#6c6}.away{color:#888}
  .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></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>