feat: enhance player login validation and update betting preview

- Added username and password validation to the player login screen, improving user feedback for input errors.
- Updated the betting preview dialog to conditionally display rebate information based on the applied instant rebate status.
- Enhanced documentation in AGENTS.md to clarify credit flow and settlement processes.
- Modified .env.example to provide clearer instructions for multi-site deployment configurations.

src/types/api/ticket.ts

@@ -53,6 +53,7 @@ export type TicketPreviewData = {
     total_rebate_amount: number;
     total_actual_deduct: number;
     total_estimated_payout: number;
+    instant_rebate_applied?: boolean;
   };
   lines: TicketPreviewLine[];
   warnings: TicketPreviewWarning[];