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/scroll-area/scroll-area-scrollbar.svelte

@@ -0,0 +1,30 @@
+<script lang="ts">
+	import { ScrollArea as ScrollAreaPrimitive } from "bits-ui";
+	import { cn, type WithoutChild } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		orientation = "vertical",
+		children,
+		...restProps
+	}: WithoutChild<ScrollAreaPrimitive.ScrollbarProps> = $props();
+</script>
+
+<ScrollAreaPrimitive.Scrollbar
+	bind:ref
+	data-slot="scroll-area-scrollbar"
+	data-orientation={orientation}
+	{orientation}
+	class={cn(
+		"data-horizontal:h-2.5 data-horizontal:flex-col data-horizontal:border-t data-horizontal:border-t-transparent data-vertical:h-full data-vertical:w-2.5 data-vertical:border-l data-vertical:border-l-transparent flex touch-none p-px transition-colors select-none",
+		className
+	)}
+	{...restProps}
+>
+	{@render children?.()}
+	<ScrollAreaPrimitive.Thumb
+		data-slot="scroll-area-thumb"
+		class="rounded-full bg-border relative flex-1"
+	/>
+</ScrollAreaPrimitive.Scrollbar>