Error toasts via use-toast never render (Toaster not mounted)

[jhodapp]

Problem

@/components/ui/use-toast's toast() writes to its store, but the use-toast <Toaster> (@/components/ui/toaster) is never mounted anywhere. The root layout (src/app/layout.tsx) mounts only the sonner <Toaster> (@/components/ui/sonner). So every toast() call from use-toast is a no-op visually.

Impact

Coaching-session panel error toasts that use use-toast never appear, e.g.:

• handleTopicCreate — "Failed to add topic"
• handleTopicEdit — "Failed to update topic"
• handleAgreementDelete — "Failed to delete agreement"

(and any other toast({ variant: "destructive", ... }) call). Toasts that use sonner (sonnerToast.*) render fine, since that Toaster is mounted.

Fix options

1. Switch the use-toast callers to sonner (sonnerToast.error(...)) — matches the mounted Toaster and the rest of the panel. (Already done for the title-save toast in Coaching session Title + collaborative Topics (frontend) #418.)
2. Or mount <Toaster/> from @/components/ui/toaster in the root layout, if both toast systems are intended.

Recommend (1) for consistency unless both systems are wanted.

Discovered while addressing #418 review comments.