feat(web): merge Entities + Graph into a single Knowledge Base page

Replaces the separate Entities/Graph nav items with one Knowledge Base
page that browses all entities as either a table or a force-graph,
scoped by ontology layer (Infrastructure/Governance/Cognition), with a
resizable browse/detail split instead of a slide-over sheet.

- New KnowledgeBase.svelte: layer tabs, view toggle, resizable
  browse/detail split (pattern from Chat.svelte's rail).
- EntityTable/EntityGraph extracted as presentational sub-components;
  their search/filter/root/depth toolbars live in the shared page
  toolbar (not the resizable pane) so they don't truncate when the
  divider is dragged narrow, and both views start flush with the
  detail pane for consistent height.
- EntityTable columns are sortable (slug/type/name/state/health).
- EntityDetailContent redesigned as a single-column list of
  collapsible sections (DetailSection.svelte), collapsed by default
  when empty; relation entries are clickable and select the entity in
  the browse pane + detail pane (and drill in-place in EntitySheet
  wherever it's used elsewhere in the app).
- api.ts: add layer filter to fetchEntities, add fetchEntityTypes for
  client-side graph layer scoping (the graph endpoint has no layer
  param).

Old hash routes (#/entities, #/graph) redirect to #/kb.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/lib/components/DetailSection.svelte

@@ -0,0 +1,34 @@
+<script lang="ts">
+  import * as Collapsible from '$lib/components/ui/collapsible'
+  import ChevronDownIcon from '@lucide/svelte/icons/chevron-down'
+  import type { Snippet } from 'svelte'
+
+  let {
+    title,
+    count,
+    defaultOpen,
+    children
+  }: {
+    title: string
+    count?: number
+    defaultOpen: boolean
+    children: Snippet
+  } = $props()
+
+  let open = $state(defaultOpen)
+</script>
+
+<Collapsible.Root bind:open class="rounded-lg border bg-card">
+  <Collapsible.Trigger class="flex w-full cursor-pointer select-none items-center justify-between gap-2 px-3 py-2 text-left hover:bg-muted/50">
+    <span class="text-sm font-medium">{title}{count !== undefined ? ` (${count})` : ''}</span>
+    <ChevronDownIcon
+      class="size-4 shrink-0 text-muted-foreground transition-transform duration-200 {open ? 'rotate-180' : ''}"
+      aria-hidden="true"
+    />
+  </Collapsible.Trigger>
+  <Collapsible.Content class="overflow-hidden data-[state=closed]:animate-out data-[state=closed]:fade-out data-[state=open]:animate-in data-[state=open]:fade-in">
+    <div class="border-t px-3 py-2.5">
+      {@render children()}
+    </div>
+  </Collapsible.Content>
+</Collapsible.Root>