feat(ui): M4 — agent activity, knowledge search, audit, correlation grouping

Add three new pages completing the control-room web UI:
- Agent activity: polls /agent-activity every 5s, filterable by type/agent
- Knowledge search: FTS over /knowledge/search with snippet + entity links
- Audit trail: browseable audit log with actor/action/entity filters

Enhanced live events page with correlation-id clustering (Groups toggle).
Added fetchAgentActivity/searchKnowledge/fetchAudit to the API client.
11 nav items now cover all planned control-room views.

web/src/lib/components/ui/dropdown-menu/dropdown-menu-sub.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+	import { DropdownMenu as DropdownMenuPrimitive } from "bits-ui";
+
+	let { open = $bindable(false), ...restProps }: DropdownMenuPrimitive.SubProps = $props();
+</script>
+
+<DropdownMenuPrimitive.Sub bind:open {...restProps} />