Replace sonner toasts with one top-centre notification stack

The app said things in two places: sonner's bottom-right toasts and the
custom top-centre chrome. This is the second of those, generalised — a
module-level store anyone can call, drawn as a stack of frosted cards that go
on their own unless raised as persistent, with room for buttons and for a
message longer than a line.

useCustomToast keeps its signatures, so the forty-odd call sites behind it and
handleError are untouched. The component and its store are byte-identical with
the portal's copy and checked by `make design-check`.

frontend/src/hooks/useCustomToast.ts

@@ -1,16 +1,20 @@
-import { toast } from "sonner"
+import { notify } from "@/lib/notificationStore"
 
+/**
+ * The two things most call sites have to say.
+ *
+ * A thin front on `notify` — the description is the whole message now, since a
+ * card that leads with "Success!" spends its first line saying nothing. Kept as
+ * a hook because that is how it is called everywhere; anything needing a
+ * severity, a button or a persistent card calls `notify` directly.
+ */
 const useCustomToast = () => {
   const showSuccessToast = (description: string) => {
-    toast.success("Success!", {
-      description,
-    })
+    notify(description, "success")
   }
 
   const showErrorToast = (description: string) => {
-    toast.error("Something went wrong!", {
-      description,
-    })
+    notify(description, "error")
   }
 
   return { showSuccessToast, showErrorToast }