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/api.ts

@@ -190,6 +190,8 @@ export interface Entity {
 export interface EntityFilters {
   type?: string
   state?: string
+  domain?: string
+  layer?: string
   q?: string
 }
 
@@ -197,6 +199,8 @@ export async function fetchEntities(filters: EntityFilters = {}): Promise<Entity
   const params = new URLSearchParams()
   if (filters.type) params.set('type', filters.type)
   if (filters.state) params.set('state', filters.state)
+  if (filters.domain) params.set('domain', filters.domain)
+  if (filters.layer) params.set('layer', filters.layer)
   if (filters.q) params.set('q', filters.q)
   params.set('limit', '200')
   const res = await fetchWithAuth(`${API}/entities?${params}`)
@@ -205,6 +209,26 @@ export async function fetchEntities(filters: EntityFilters = {}): Promise<Entity
   return data.items ?? []
 }
 
+export type OntologyLayer = 'meta' | 'infrastructure' | 'governance' | 'cognition'
+
+export interface EntityType {
+  name: string
+  parent_type?: string | null
+  is_abstract: boolean
+  domain: string
+  layer: OntologyLayer
+  description?: string | null
+}
+
+// The graph endpoint has no layer param, so callers build a type→layer map from
+// this to scope the graph client-side (the entities table filters server-side).
+export async function fetchEntityTypes(): Promise<EntityType[]> {
+  const res = await fetchWithAuth(`${API}/ontology`)
+  if (!res.ok) return []
+  const data = await res.json()
+  return data.entity_types ?? []
+}
+
 export interface EventFilters {
   type?: string
   severity?: string