Add the Secrets and Alerts screens

Two backends that had no way in: secrets could only be set through the API,
and alerting could only be configured by hand-editing its file. Both are
engine-wide operator settings rather than personal ones, so they get sidebar
entries of their own instead of tabs under the per-user Settings — and the
missing-secret error now names the place the page actually is.

frontend/src/components/Sidebar/AppSidebar.tsx

@@ -1,5 +1,7 @@
 import {
+  Bell,
   Home,
+  KeyRound,
   LayoutDashboard,
   LogOut,
   Settings,
@@ -23,6 +25,10 @@ const baseItems: Item[] = [
   { icon: Home, title: "Home", path: "/" },
   { icon: Workflow, title: "Flows", path: "/flows" },
   { icon: LayoutDashboard, title: "Dashboards", path: "/dashboards" },
+  // Both are engine-wide operator settings rather than personal ones, so they
+  // sit here and not among the per-user tabs under Settings.
+  { icon: KeyRound, title: "Secrets", path: "/secrets" },
+  { icon: Bell, title: "Alerts", path: "/alerts" },
 ]
 
 export function AppSidebar() {