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/app/admin/stops/page.tsx

@@ -1,5 +1,6 @@
 import { pool } from "@/lib/db";
 import { getAdminUser } from "@/lib/admin-permissions";
+import { getActiveBrandId } from "@/lib/brand-scope";
 import AdminAccessDenied from "@/components/admin/AdminAccessDenied";
 import { PageHeader } from "@/components/admin/design-system";
 import { redirect } from "next/navigation";
@@ -26,11 +27,18 @@ export default async function AdminStopsPage({ searchParams }: PageProps) {
   let error: string | null = null;
   let totalCount = 0;
 
+  // Resolve active brand from cookie/URL (respects platform_admin "All brands" = null)
+  let activeBrandId: string | null = null;
   try {
-    // Platform admin sees all stops, brand admin sees their brand's stops
-    const brandCondition = adminUser.brand_id
-      ? `brand_id = '${adminUser.brand_id}'`
-      : "1=1";
+    activeBrandId = await getActiveBrandId(adminUser);
+
+    // If brand-scoped (not platform_admin) and no active brand, restrict query
+    const brandCondition =
+      activeBrandId
+        ? `brand_id = '${activeBrandId}'`
+        : adminUser.role === "platform_admin"
+          ? "1=1"
+          : `brand_id IN ('${(adminUser.brand_ids ?? []).join("','")}')`;
 
     const countResult = await pool.query<{ count: string }>(
       `SELECT count(*) as count FROM stops WHERE ${brandCondition}`
@@ -104,9 +112,9 @@ export default async function AdminStopsPage({ searchParams }: PageProps) {
             </div>
             <h3 className="font-display text-2xl font-medium text-stone-950">No stops found</h3>
             <p className="mt-2 text-stone-500">
-              {adminUser.brand_id
-                ? "No stops for your brand yet. Upload a schedule or add stops manually."
-                : "No stops in the database. Import the Tuxedo Tour schedule."}
+              {activeBrandId
+                ? "No stops for the selected brand. Upload a schedule or add stops manually."
+                : "No stops in the database."}
             </p>
             <div className="mt-6 flex justify-center gap-3">
               <a