feat(events): add recurring scope toast

[tyler-dane]

Summary

• apply recurring occurrence edits and deletes immediately to only that instance
• offer keyboard-first This & following (1) and All (2) promotion from a short-lived toast
• preserve optimistic updates, undo/redo, and serialized writes without widening unrelated changes

Validation

• bun test:web
• bun run type-check
• bun run lint