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/actions/brands.ts

@@ -0,0 +1,65 @@
+import "server-only";
+
+import { getAdminUser } from "@/lib/admin-permissions";
+import { svcHeaders } from "@/lib/svc-headers";
+
+export type BrandListItem = {
+  id: string;
+  name: string;
+  slug: string;
+  logo_url: string | null;
+};
+
+/**
+ * Returns the list of brands the current admin user can act in.
+ *
+ * - platform_admin: all brands (queried directly from the `brands` table)
+ * - everyone else: brands in `adminUser.brand_ids`
+ * - empty array for unauthenticated / no-access admins
+ *
+ * This is a plain async function (not a server action) so it can be called
+ * from server components and server actions without the "use server" wrapper.
+ * The BrandSelector client component receives the result as a prop.
+ */
+export async function listBrandsForAdmin(): Promise<BrandListItem[]> {
+  const adminUser = await getAdminUser();
+  if (!adminUser) return [];
+
+  const supabaseUrl = process.env.NEXT_PUBLIC_SUPABASE_URL;
+  const serviceKey = process.env.SUPABASE_SERVICE_ROLE_KEY;
+  if (!supabaseUrl || !serviceKey) return [];
+
+  if (adminUser.role === "platform_admin") {
+    try {
+      const res = await fetch(
+        `${supabaseUrl}/rest/v1/brands?select=id,name,slug,logo_url&order=name`,
+        { headers: svcHeaders(serviceKey), cache: "no-store" }
+      );
+      if (!res.ok) return [];
+      const data = await res.json();
+      return Array.isArray(data) ? data : [];
+    } catch {
+      return [];
+    }
+  }
+
+  if (adminUser.brand_ids.length === 0) return [];
+
+  // Use PostgREST `in` filter. The brand_ids are UUIDs, so the quoting
+  // pattern in the spec is safe; the inner quotes are required by PostgREST
+  // for UUID literals.
+  const filter = `id=in.(${adminUser.brand_ids
+    .map((id) => `"${id}"`)
+    .join(",")})`;
+  try {
+    const res = await fetch(
+      `${supabaseUrl}/rest/v1/brands?${filter}&select=id,name,slug,logo_url&order=name`,
+      { headers: svcHeaders(serviceKey), cache: "no-store" }
+    );
+    if (!res.ok) return [];
+    const data = await res.json();
+    return Array.isArray(data) ? data : [];
+  } catch {
+    return [];
+  }
+}