feat: add payment management improvements and reminder emails

- Add option to approve/reject payments without sending notification emails
  (checkbox in review popup, default enabled)
- Add payment reminder email template and send functionality
- Track when reminder emails are sent (reminderSentAt field)
- Display reminder sent timestamp in payment review popup
- Make payment review popup scrollable for better UX
- Add payment-reminder template to email system (available in admin emails)

frontend/src/lib/api.ts

@@ -218,16 +218,21 @@ export const paymentsApi = {
       body: JSON.stringify(data),
     }),
   
-  approve: (id: string, adminNote?: string) =>
+  approve: (id: string, adminNote?: string, sendEmail: boolean = true) =>
     fetchApi<{ payment: Payment; message: string }>(`/api/payments/${id}/approve`, {
       method: 'POST',
-      body: JSON.stringify({ adminNote }),
+      body: JSON.stringify({ adminNote, sendEmail }),
     }),
   
-  reject: (id: string, adminNote?: string) =>
+  reject: (id: string, adminNote?: string, sendEmail: boolean = true) =>
     fetchApi<{ payment: Payment; message: string }>(`/api/payments/${id}/reject`, {
       method: 'POST',
-      body: JSON.stringify({ adminNote }),
+      body: JSON.stringify({ adminNote, sendEmail }),
+    }),
+  
+  sendReminder: (id: string) =>
+    fetchApi<{ message: string; reminderSentAt?: string }>(`/api/payments/${id}/send-reminder`, {
+      method: 'POST',
     }),
   
   updateNote: (id: string, adminNote: string) =>
@@ -502,6 +507,7 @@ export interface Payment {
   paidAt?: string;
   paidByAdminId?: string;
   adminNote?: string;
+  reminderSentAt?: string; // When payment reminder email was sent
   createdAt: string;
   updatedAt: string;
 }