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/hooks/is-mobile.svelte.ts

@@ -0,0 +1,9 @@
+import { MediaQuery } from "svelte/reactivity";
+
+const DEFAULT_MOBILE_BREAKPOINT = 768;
+
+export class IsMobile extends MediaQuery {
+	constructor(breakpoint: number = DEFAULT_MOBILE_BREAKPOINT) {
+		super(`max-width: ${breakpoint - 1}px`);
+	}
+}