Replaces our custom `<GlobalSnackbar />` (MUI Snackbar + event emitter)
with [`sonner`](https://github.com/emilkowalski/sonner). Deletes
`GlobalSnackbar/`, the custom event emitter infra, and migrates ~80
source files to `toast.success()` / `toast.error()` from `sonner`.
- ~47 error toasts now surface API error detail via
`getErrorDetail(error)` in the toast description, not just a generic
message. Coincides with #22229.
- Toast messages follow an `{Action} "{entity}" {result}.` format (e.g.
`User "alice" suspended successfully.`) since toasts persist across
navigation now.
- 17 uses of `toast.promise()` for loading → success → error lifecycle.
- Some toasts include action buttons for quick navigation (e.g. "View
task", "View template").
- Multiple toasts can stack and display simultaneously.
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Co-authored-by: Kayla はな <mckayla@hey.com>
Kayla はな