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/src/lib/api.ts

@@ -155,3 +155,19 @@ export function fetchAllUsers(token: string, params: Record<string, string> = {}
 export function fetchAllPayments(token: string, params: Record<string, string> = {}): Promise<any[]> {
   return fetchAllPages("/api/payments", token, params);
 }
+
+// Creates a Yoco checkout for a registration's full remaining outstanding balance.
+// Omitting `amount` makes the backend charge the full remainder rather than a partial amount —
+// partial-amount checkouts are only ever created from the supervisor payments dashboard.
+export function createFullPaymentCheckout(token: string, registrationId: string): Promise<{ redirectUrl?: string; priceUpdated?: boolean; newTotal?: number; message?: string }> {
+  return apiFetch("/api/payments/yoco-checkout", {
+    method: "POST",
+    authToken: token,
+    body: {
+      registrationId,
+      successUrl: window.location.origin + "/payment/success",
+      cancelUrl: window.location.origin + "/payment/cancel",
+      failureUrl: window.location.origin + "/payment/failure",
+    },
+  });
+}