fix: add brand picker to sidebar, use getActiveBrandId in stops page

- AdminSidebar now renders BrandSelector when brands are available
- Stops page uses getActiveBrandId() instead of adminUser.brand_id
  directly, so platform admins see all stops and brand selection
  via the sidebar picker works correctly
- BrandSelector already existed and was wired up in the layout,
  just never rendered in the sidebar

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 { signOutAction } from "@/actions/auth-actions";
+import BrandSelector from "@/components/admin/BrandSelector";
 
 // Elegant warm sidebar design
 // Colors: parchment 100 bg, soft linen text, powder petal accent
@@ -470,15 +471,25 @@ export default function AdminSidebar({
           </ul>
         </nav>
 
-        {/* Bottom: role + sign out */}
-        <div 
-          className="px-4 py-5 border-t flex-shrink-0" 
+        {/* Bottom: brand picker + role + sign out */}
+        <div
+          className="px-4 py-5 border-t flex-shrink-0 space-y-3"
           style={{ borderColor: "rgba(208, 203, 180, 0.2)" }}
         >
+          {/* Brand selector — only show when admin has access to brands */}
+          {brands && brands.length > 0 && (
+            <BrandSelector
+              brands={brands}
+              activeBrandId={activeBrandId ?? null}
+              showAllBrandsOption={userRole === "platform_admin"}
+              isMultiBrandAdmin={(brandIds?.length ?? 0) > 1}
+            />
+          )}
+
           {roleLabel && (
-            <div 
-              className="px-3 py-2.5 mb-3 rounded-xl border" 
-              style={{ 
+            <div
+              className="px-3 py-2.5 rounded-xl border"
+              style={{
                 backgroundColor: "rgba(208, 203, 180, 0.1)",
                 borderColor: "rgba(208, 203, 180, 0.2)"
               }}