feat: tenants admin surface

Adds the first admin screen — /tenants — listing tenants from
GET /api/v1/admin/tenants with search, status badges, plan, created
date, and a per-row menu with suspend / activate / deactivate actions.

Hand-typed shapes in app/lib/arcadia/tenants.ts because arcadia's admin
endpoints aren't yet covered by /api/openapi (same 'ok'-placeholder
issue documented in lib-arcadia-client/scripts/sync-spec.mjs). When
the spec gains coverage, switch to arcadia.typed.GET(...) and drop the
manual types.

Trims the inherited consumer-app sidenav (Resources / Assistant / AI /
Library) down to admin-shaped items: Overview, Tenants, Audit log,
Settings. The unused route files stay in place; they just aren't linked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/components/layout/app-shell.tsx

@@ -7,6 +7,7 @@ import {
   LayoutDashboard,
   Boxes,
   Activity,
+  Building2,
   Sparkles,
   Bot,
   BookOpen,
@@ -86,11 +87,8 @@ type NavItem = {
 
 const navItems: NavItem[] = [
   { to: "/", icon: LayoutDashboard, label: "Overview", end: true },
-  { to: "/resources", icon: Boxes, label: "Resources" },
-  { to: "/activity", icon: Activity, label: "Activity" },
-  { to: "/assistant", icon: Sparkles, label: "Assistant" },
-  { to: "/ai", icon: Bot, label: "AI" },
-  { to: "/library", icon: BookOpen, label: "Library" },
+  { to: "/tenants", icon: Building2, label: "Tenants" },
+  { to: "/activity", icon: Activity, label: "Audit log" },
   { to: "/settings", icon: Settings, label: "Settings" },
   // CREMA:NAV-ITEMS
 ]