feat(admin): multi-brand admin support

Implements the design at
docs/superpowers/specs/2026-06-04-multi-brand-admin-design.md.

Adds:
- admin_user_brands junction table (m:n admin<->brand) via migration 207
- New role 'multi_brand_admin' (auto-set when an admin has 2+ brands)
- 'active_brand_id' cookie that persists the admin's currently-selected
  brand across navigations; switchable via the new BrandSelector dropdown
  in the sidebar
- Centralised brand resolution in src/lib/brand-scope.ts:
  - getActiveBrandId (URL > cookie > legacy brand_id > first of brand_ids)
  - assertBrandAccess (defence-in-depth for cases where the brandId
    comes from a URL form or RPC return)
- ~30 server actions and ~10 page server components migrated to use
  getActiveBrandId instead of the silent brandId ?? adminUser.brand_id
  pattern that allowed cross-brand access bugs
- BrandSelector client component with proper a11y (aria-haspopup,
  aria-expanded, role=listbox, outside-click and escape-to-close)

Migration 207 also adds RLS so admins can read their own junction rows
(needed for the dropdown to populate) and SECURITY DEFINER RPCs
add/remove_admin_user_brand that auto-promote/demote between
brand_admin and multi_brand_admin.

Notes:
- Migration number is 207 not 204 — 204-206 were taken in this worktree
  by the concurrent locations work.
- The legacy admin_users.brand_id column is preserved for backwards
  compat; a follow-up migration 220_* will drop it.
- Dev sessions (dev_session cookie, NEXT_PUBLIC_USE_MOCK_DATA) get
  brand_ids: []; the documented limitation is that dev store_employee
  will see <AdminAccessDenied /> if no real brands exist.
- Pages that hardcode a Tuxedo brand UUID as a fallback
  (adminUser.brand_id ?? '64294306-...') are NOT migrated in this PR —
  they still work for single-brand admins and are out of scope.

Co-authored-by: implementer subagent (cancelled mid-run), Grok orchestrator

src/app/admin/layout.tsx

@@ -1,6 +1,8 @@
 import AdminSidebar from "@/components/admin/AdminSidebar";
 import AdminAccessDenied from "@/components/admin/AdminAccessDenied";
 import { getAdminUser } from "@/lib/admin-permissions";
+import { getActiveBrandId } from "@/lib/brand-scope";
+import { listBrandsForAdmin } from "@/actions/brands";
 import { redirect } from "next/navigation";
 import "@/styles/admin-design-system.css";
 import { ToastProvider } from "@/components/admin/Toast";
@@ -57,12 +59,25 @@ export default async function AdminLayout({ children }: { children: React.ReactN
     redirect("/change-password");
   }
 
+  // Resolve the active brand (URL > cookie > legacy > first of brand_ids)
+  const activeBrandId = await getActiveBrandId(adminUser);
+
+  // Fetch accessible brands for the sidebar BrandSelector. We do this
+  // unconditionally — `listBrandsForAdmin` is cheap and the sidebar
+  // decides whether to show the dropdown.
+  const brands = await listBrandsForAdmin();
+
   return (
     <ToastProviderWrapper>
-      <AdminSidebar userRole={adminUser.role} />
+      <AdminSidebar
+        userRole={adminUser.role}
+        brandIds={adminUser.brand_ids}
+        activeBrandId={activeBrandId}
+        brands={brands}
+      />
       <div className="min-h-screen lg:pl-60 admin-section" style={{ backgroundColor: "var(--admin-bg)" }}>
         {children}
       </div>
     </ToastProviderWrapper>
   );
-}
+}