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/components/admin/AdminSidebar.tsx

@@ -5,6 +5,7 @@ import Link from "next/link";
 import { usePathname } from "next/navigation";
 import { useRouter } from "next/navigation";
 import { supabase } from "@/lib/supabase";
+import BrandSelector from "./BrandSelector";
 
 // Elegant warm sidebar design
 // Colors: parchment 100 bg, soft linen text, powder petal accent
@@ -217,9 +218,12 @@ const ICON_MAP: Record<string, React.ReactNode> = {
 
 type SidebarProps = {
   userRole?: string | null;
+  brandIds?: string[];
+  activeBrandId?: string | null;
+  brands?: Array<{ id: string; name: string; slug: string; logo_url: string | null }>;
 };
 
-export default function AdminSidebar({ userRole }: SidebarProps) {
+export default function AdminSidebar({ userRole, brandIds, activeBrandId, brands }: SidebarProps) {
   const pathname = usePathname();
   const router = useRouter();
   const [mobileOpen, setMobileOpen] = useState(false);
@@ -230,9 +234,15 @@ export default function AdminSidebar({ userRole }: SidebarProps) {
 
   const roleLabel = userRole === "platform_admin" ? "Platform Admin"
     : userRole === "brand_admin" ? "Brand Admin"
+    : userRole === "multi_brand_admin" ? "Multi-Brand Manager"
     : userRole === "store_employee" ? "Store Employee"
     : null;
 
+  const isMultiBrandAdmin = userRole === "multi_brand_admin";
+  const showAllBrandsOption = userRole === "platform_admin";
+  const showBrandSelector =
+    brands && (showAllBrandsOption || (brandIds && brandIds.length >= 2));
+
   const isActive = useCallback((href: string) => {
     if (href === "/admin") return pathname === "/admin";
     return pathname.startsWith(href);
@@ -394,6 +404,24 @@ export default function AdminSidebar({ userRole }: SidebarProps) {
           </Link>
         </div>
 
+        {/* Brand selector (multi-brand admins + platform_admin) */}
+        {showBrandSelector && (
+          <div className="px-4 py-3 border-b flex-shrink-0" style={{ borderColor: "rgba(208, 203, 180, 0.2)" }}>
+            <p
+              className="text-[10px] font-semibold uppercase tracking-widest mb-1.5 px-1"
+              style={{ color: "rgba(195, 195, 193, 0.6)" }}
+            >
+              Active Brand
+            </p>
+            <BrandSelector
+              brands={brands!}
+              activeBrandId={activeBrandId ?? null}
+              showAllBrandsOption={showAllBrandsOption}
+              isMultiBrandAdmin={isMultiBrandAdmin}
+            />
+          </div>
+        )}
+
         {/* Nav links with keyboard navigation */}
         <nav className="flex-1 overflow-y-auto overflow-x-hidden px-3 py-4 scrollbar-thin">
           <ul className="space-y-1" role="list">