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/orders/page.tsx

@@ -1,5 +1,6 @@
 import AdminOrdersPanel from "@/components/admin/AdminOrdersPanel";
 import { getAdminUser } from "@/lib/admin-permissions";
+import { getActiveBrandId } from "@/lib/brand-scope";
 import { getAdminOrders } from "@/actions/orders";
 import AdminAccessDenied from "@/components/admin/AdminAccessDenied";
 import { PageHeader } from "@/components/admin/design-system";
@@ -17,13 +18,19 @@ export default async function AdminOrdersPage() {
     redirect("/admin/pickup");
   }
 
+  const activeBrandId = await getActiveBrandId(adminUser);
+  // Platform admin can browse all brands' orders; everyone else must have a brand
+  if (!activeBrandId && adminUser.role !== "platform_admin") {
+    return <AdminAccessDenied message="You don't have access to any brand." />;
+  }
+
   const { orders, stops } = await getAdminOrders();
 
-  const brandStops = adminUser?.brand_id
-    ? stops.filter((s) => s.brand_id === adminUser.brand_id)
+  const brandStops = activeBrandId
+    ? stops.filter((s) => s.brand_id === activeBrandId)
     : stops;
 
-  const brandOrders = adminUser?.brand_id
+  const brandOrders = activeBrandId
     ? orders.filter(
         (o) =>
           o.stops && brandStops.some((s) => s.id === o.stop_id)
@@ -41,8 +48,8 @@ export default async function AdminOrdersPage() {
       .order("name")
       .limit(200);
 
-    if (adminUser?.brand_id) {
-      prodQuery = prodQuery.eq("brand_id", adminUser.brand_id);
+    if (activeBrandId) {
+      prodQuery = prodQuery.eq("brand_id", activeBrandId);
     }
 
     const { data: prods } = await prodQuery;
@@ -80,7 +87,7 @@ export default async function AdminOrdersPage() {
             initialOrders={brandOrders}
             initialStops={brandStops}
             initialProducts={brandProducts}
-            brandId={adminUser?.brand_id ?? null}
+            brandId={activeBrandId}
           />
         </div>
       </div>