[FE-29] Build unified Toast abstraction wrapping Sonner to replace dual-library usage

[devwums]

Problem

The codebase imports both react-hot-toast and sonner — two competing toast libraries coexist with inconsistent styling and usage patterns.

Proposed Solution

Create frontend/cntr/Toast/toast.ts — a unified toast abstraction wrapping sonner. Exports: showSuccess(message: string): void, showError(message: string): void, showInfo(message: string): void, showLoading(message: string): string | number, dismissToast(id: string | number): void. All 5 functions route through Sonner with ManageHub styling defaults. All implementation must live inside frontend/cntr/.

Acceptance Criteria

• File at frontend/cntr/Toast/toast.ts
• All 5 functions exported as named exports
• All use sonner — not react-hot-toast
• showError has no auto-dismiss (duration: Infinity)
• showSuccess and showInfo auto-dismiss after 4000ms
• Test in frontend/cntr/Toast/toast.test.ts verifying each function calls the correct sonner method