search: add Search section calling arcadia-search admin sidecar

New /search route manages tenants and corpora on the arcadia-search
box via its privileged /admin/* surface (default :7801) — KPI tiles,
flat tenant×corpus table with Rebuild / Edit config / Delete
actions, New tenant / New corpus dialogs, and a Restart service
button. New app/lib/search-admin.ts wraps the bearer-token fetch.

Configured by VITE_ARCADIA_SEARCH_ADMIN_URL +
VITE_ARCADIA_SEARCH_ADMIN_TOKEN; the route renders a warning banner
when the token is unset. Token ships in the client bundle — fine for
this internal tool, called out in CLAUDE.md and the source comments.

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

app/components/layout/app-shell.tsx

@@ -31,6 +31,7 @@ import {
   ShieldCheck,
   Megaphone,
   AlertOctagon,
+  SearchCode,
   // CREMA:NAV-ICONS
 } from "lucide-react"
 
@@ -112,6 +113,7 @@ const navItems: NavItem[] = [
   { to: "/announcements", icon: Megaphone, label: "Announcements" },
   { to: "/status-page", icon: AlertOctagon, label: "Status page" },
   { to: "/activity", icon: Activity, label: "Audit log" },
+  { to: "/search", icon: SearchCode, label: "Search" },
   { to: "/ai", icon: Bot, label: "AI" },
   { to: "/settings", icon: Settings, label: "Settings" },
   // CREMA:NAV-ITEMS