Admin-only Accounting tab + platform COA consolidation

Phase 1: gate the Accounting sidebar item and /dashboard/accounting route
behind isAdmin via a RequireAdmin guard; Financial Reports stay visible.

Phase 2: platform associations now read the Chart of Accounts from
accounting.accounts (single source) instead of public.chart_of_accounts.
Shared fetchChartOfAccounts() normalizes both sources; central COA hooks
and ChartOfAccountsDropdown route through it (reads only, no migration).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/RequireAdmin.tsx

@@ -0,0 +1,23 @@
+import { Navigate } from "react-router-dom";
+import { useAuth } from "@/contexts/AuthContext";
+
+/**
+ * Route guard for admin-only areas (e.g. the Accounting platform).
+ * Shows a spinner while auth resolves, then redirects non-admins to the
+ * dashboard. Admin status respects "view as" (uses effective `isAdmin`).
+ */
+export function RequireAdmin({ children }: { children: React.ReactNode }) {
+  const { loading, isAdmin } = useAuth();
+
+  if (loading) {
+    return (
+      <div className="flex min-h-[40vh] items-center justify-center">
+        <div className="h-8 w-8 animate-spin rounded-full border-4 border-primary border-t-transparent" />
+      </div>
+    );
+  }
+
+  if (!isAdmin) return <Navigate to="/dashboard" replace />;
+
+  return <>{children}</>;
+}