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-button.svelte

@@ -0,0 +1,39 @@
+<script lang="ts">
+	import { cn, type WithElementRef } from "$lib/utils.js";
+	import type { Snippet } from "svelte";
+	import type { HTMLAnchorAttributes } from "svelte/elements";
+
+	let {
+		ref = $bindable(null),
+		children,
+		child,
+		class: className,
+		size = "md",
+		isActive = false,
+		...restProps
+	}: WithElementRef<HTMLAnchorAttributes> & {
+		child?: Snippet<[{ props: Record<string, unknown> }]>;
+		size?: "sm" | "md";
+		isActive?: boolean;
+	} = $props();
+
+	const mergedProps = $derived({
+		class: cn(
+			"text-sidebar-foreground ring-sidebar-ring hover:bg-sidebar-accent hover:text-sidebar-accent-foreground active:bg-sidebar-accent active:text-sidebar-accent-foreground [&>svg]:text-sidebar-accent-foreground data-active:bg-sidebar-accent data-active:text-sidebar-accent-foreground h-7 gap-2 rounded-md px-2 focus-visible:ring-2 data-[size=md]:text-sm data-[size=sm]:text-xs [&>svg]:size-4 flex min-w-0 -translate-x-px items-center overflow-hidden outline-hidden group-data-[collapsible=icon]:hidden disabled:pointer-events-none disabled:opacity-50 aria-disabled:pointer-events-none aria-disabled:opacity-50 [&>span:last-child]:truncate [&>svg]:shrink-0",
+			className
+		),
+		"data-slot": "sidebar-menu-sub-button",
+		"data-sidebar": "menu-sub-button",
+		"data-size": size,
+		"data-active": isActive,
+		...restProps,
+	});
+</script>
+
+{#if child}
+	{@render child({ props: mergedProps })}
+{:else}
+	<a bind:this={ref} {...mergedProps}>
+		{@render children?.()}
+	</a>
+{/if}