Simplify self-service payments to full-amount checkout, add manual-page payment tab

Self-service "pay now" flows (registration success + user dashboard) now go
straight to a Yoco checkout for the full outstanding balance instead of
prompting for a partial amount; that page has been removed. Partial-amount
payment links remain supervisor/admin-only via the Payments dashboard.

Also adds a "Record Payment" tab to the supervisor manual registration page,
pre-filled with the most recently created registration, so staff can capture
a payment right after registering someone without leaving the page.

frontend/README.md

@@ -164,7 +164,6 @@ frontend/src/
 |-------|-------------|
 | `/dashboard/user` | My registrations + tickets overview |
 | `/dashboard/user/profile` | Edit profile, notification preferences |
-| `/dashboard/user/pay` | Pay outstanding balance |
 | `/dashboard/user/donate` | Make a donation |
 | `/dashboard/user/forms` | Complete registration forms |
 | `/dashboard/user/reset-password` | Change password (authenticated) |
@@ -255,7 +254,7 @@ A "Select Email/both" or "Select WhatsApp/both" quick-select button is available
 5. Backend reconciles payment, updates registration status, generates and emails tickets
 6. User sees `/payment/success` and tickets appear in their dashboard
 
-Outstanding balances can be paid at any time from `/dashboard/user/pay`.
+Outstanding balances can be paid at any time from the "Make payment" button on `/dashboard/user` — this always creates a checkout for the full remaining balance. Partial-amount payment links can only be generated by a supervisor/admin from `/dashboard/supervisor/payments`.
 
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