feat: add discount management and PDF export for transmittals

- Create Discount model, migration, and Filament resource with relation to Client
- Add PDF export functionality for transmittals using DomPDF
- Include discount type selection in sales transactions
- Fix account filtering logic in expense resource
- Update export job to generate PDF instead of Excel

app/Models/Discount.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class Discount extends Model
+{
+    protected $fillable = ['discount'];
+}