Introduces a runes-based toast module (`$lib/toast.svelte.ts`) with
success/info/warning/error variants, auto-dismiss, optional action
button, plus a `Toaster` viewport mounted in the root layout.
Migrates the lone `alert(e.message)` call site (stores delete) to
`toast.error()`. Backlog updated to remove the now-completed
foundational item and rewrite dependent items to reference the
shipped API.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>