Fix defer/edit; remove area; improve UPI button

- Removed the 'area' field (Venghlui, Chhimveng) entirely from data structures,
  seed data, and UI – no longer needed.
- Fixed defer checkbox: now uses a stable entryId (string) instead of relying on
  createdAt timestamps, making it work reliably in both local storage and Firebase.
- Fixed edit button: same entryId fix ensures the edit modal opens correctly and
  updates the right entry.
- Redesigned "Pay Now" button: full-width, WhatsApp‑green, with phone emoji and
  subtle shadow for better visibility and UX.
- Updated UPI instruction text from "payment will be confirmed by admin" to
  "Send screenshot to Admin via WhatsApp" to clarify the workflow.
- Ensured all ledger entries (charges, payments, forgiveness) now store a unique
  entryId for consistent referencing in all CRUD actions.

Author: enga018