swarm-ui: rename OverviewPage/'overview' to HivesPage/'hives'

Closes #3460.

Renamed for accuracy — the page/nav item is the hive roster, not a
general dashboard overview. Nav label, page title, component name,
and file all renamed together so the internal name doesn't drift from
what's displayed (a component still called OverviewPage under a
'hives' nav label would be exactly the kind of stale prior-art that
makes the next contributor search harder, not less).

frontend/packages/swarm-ui/src/shell/Shell.tsx

@@ -19,7 +19,7 @@ import { LinksMenu } from './LinksMenu.js';
 import './Shell.css';
 
 const NAV_ITEMS: { href: string; label: string }[] = [
-  { href: '/', label: 'overview' },
+  { href: '/', label: 'hives' },
   { href: '/create-agent', label: 'new agent' },
   { href: '/jobs', label: 'jobs' },
   { href: '/components', label: 'components' },