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/routes.ts

@@ -10,5 +10,6 @@ export default [
   route("settings", "routes/settings.tsx"),
   route("profile", "routes/profile.tsx"),
   route("login", "routes/login.tsx"),
+  route("tenants", "routes/tenants.tsx"),
   // CREMA:ROUTES
 ] satisfies RouteConfig