feat(timeslots): confirm-and-message flow for timeslot deletion

[choden-dev]

Summary

Builds on the timeslot-deletion notification (#97) to give admins control over the message customers receive. Instead of an automatic, generic email on delete, deleting a timeslot now goes through a deliberate confirmation flow where the admin can write a personalised message.

Behaviour

• On the Timeslot edit view, a "Delete timeslot & notify customers" button opens a confirmation modal.
• The modal shows how many orders will be notified and a textarea pre-filled with an editable default apology (mentioning the slot's date/time). The admin can tweak, replace, or clear it.
• On confirm, an admin API route:
  1. clears each affected order's pickupTimeslot so the customer can re-book,
  2. emails every booked customer (including the admin's note as an "A note from us" section),
  3. deletes the timeslot.
• The admin is redirected back to the timeslots list with a summary of how many customers were notified.

Changes

• collections/Timeslots.ts: removed the automatic afterDelete notification hook; registered DeleteTimeslotButton via admin.components.edit.beforeDocumentControls.
• components/admin/DeleteTimeslotButton.tsx (new): client component with confirmation modal, affected-order count, and editable message box.
• app/api/admin/timeslots/[id]/delete/route.ts (new): admin-authenticated GET (affected count + label) and POST (clear references, notify, delete).
• lib/email.ts: sendTimeslotDeletedEmail now accepts an optional customMessage.
• lib/templates/timeslotDeleted.pug: renders the admin's message as a highlighted "A note from us" section (falls back to the standard apology when empty). No em dashes.
• app/(payload)/admin/importMap.js: registered the new component.

Notes

• Because the auto afterDelete hook was removed, notification now happens only through this new button. Payload's default delete controls will not notify customers.

Testing

• tsc --noEmit passes for changed files.
• biome check clean.
• Pug template renders correctly both with and without a custom message.