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/sidebar/sidebar-menu-sub.svelte

@@ -0,0 +1,21 @@
+<script lang="ts">
+	import { cn, type WithElementRef } from "$lib/utils.js";
+	import type { HTMLAttributes } from "svelte/elements";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		children,
+		...restProps
+	}: WithElementRef<HTMLAttributes<HTMLUListElement>> = $props();
+</script>
+
+<ul
+	bind:this={ref}
+	data-slot="sidebar-menu-sub"
+	data-sidebar="menu-sub"
+	class={cn("border-sidebar-border mx-3.5 translate-x-px gap-1 border-l px-2.5 py-0.5 group-data-[collapsible=icon]:hidden flex min-w-0 flex-col", className)}
+	{...restProps}
+>
+	{@render children?.()}
+</ul>