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/analytics.ts

@@ -1,6 +1,7 @@
 "use server";
 
 import { getAdminUser } from "@/lib/admin-permissions";
+import { getActiveBrandId } from "@/lib/brand-scope";
 import { svcHeaders } from "@/lib/svc-headers";
 
 const supabaseUrl = process.env.NEXT_PUBLIC_SUPABASE_URL!;
@@ -96,7 +97,7 @@ async function brandScopedRPC<T>(
   const adminUser = await getAdminUser();
   if (!adminUser) throw new Error("Not authenticated");
 
-  const brandId = adminUser.brand_id ?? null;
+  const brandId = await getActiveBrandId(adminUser);
 
   const response = await fetch(`${supabaseUrl}/rest/v1/rpc/${rpcName}`, {
     method: "POST",
@@ -232,7 +233,7 @@ export async function getRecentOrders(limit: number = 10): Promise<RecentOrder[]
     const adminUser = await getAdminUser();
     if (!adminUser) throw new Error("Not authenticated");
 
-    const brandId = adminUser.brand_id ?? null;
+    const brandId = await getActiveBrandId(adminUser);
 
     const params = new URLSearchParams({
       select: "id,customer_name,subtotal,status,created_at,fulfillment",
@@ -293,7 +294,7 @@ export async function getConversionFunnel(): Promise<ConversionFunnel[]> {
     const adminUser = await getAdminUser();
     if (!adminUser) throw new Error("Not authenticated");
 
-    const brandId = adminUser.brand_id ?? null;
+    const brandId = await getActiveBrandId(adminUser);
 
     const params = new URLSearchParams({
       select: "id,status",