Feat: Implementado cálculo de ISR en nóminas

- Agregado campo isr_table_id en tabla months para seleccionar tabla ISR por mes
- Creado servicio IsrCalculator para calcular ISR mensual y quincenal
- Modificado CommissionCalculator para descontar ISR del total a pagar
- Agregado selector de tabla ISR en formulario de crear/editar mes
- Actualizada vista de meses para mostrar tabla ISR asignada
- Actualizados reportes mensual y quincenal para mostrar ISR descontado

app/Services/IsrCalculator.php

@@ -0,0 +1,65 @@
+<?php
+
+namespace App\Services;
+
+use App\Models\IsrBracket;
+use App\Models\IsrTable;
+
+class IsrCalculator
+{
+    /**
+     * Calcular ISR mensual dado un ingreso base
+     * Retorna: ['isr' => float, 'bracket' => array|null, 'effective_rate' => float]
+     */
+    public static function calculateMonthly(float $baseIncome, ?IsrTable $isrTable): array
+    {
+        if (!$isrTable || $baseIncome <= 0) {
+            return [
+                'isr' => 0,
+                'bracket' => null,
+                'effective_rate' => 0,
+                'taxable_income' => 0,
+            ];
+        }
+
+        $brackets = $isrTable->brackets()->orderBy('lower_limit')->get();
+        
+        foreach ($brackets as $bracket) {
+            $upperLimit = $bracket->upper_limit;
+            
+            // Si es el último bracket (upper_limit es null = "En adelante")
+            if ($upperLimit === null || $baseIncome <= $upperLimit) {
+                // Calcular ISR: Cuota Fija + (Excedente × Tasa)
+                $excedent = $baseIncome - $bracket->lower_limit;
+                $isr = $bracket->fixed_fee + ($excedent * $bracket->rate / 100);
+                
+                return [
+                    'isr' => round($isr, 2),
+                    'bracket' => [
+                        'lower_limit' => $bracket->lower_limit,
+                        'upper_limit' => $bracket->upper_limit,
+                        'fixed_fee' => $bracket->fixed_fee,
+                        'rate' => $bracket->rate,
+                    ],
+                    'effective_rate' => round(($isr / $baseIncome) * 100, 2),
+                    'taxable_income' => round($baseIncome, 2),
+                ];
+            }
+        }
+        
+        return [
+            'isr' => 0,
+            'bracket' => null,
+            'effective_rate' => 0,
+            'taxable_income' => 0,
+        ];
+    }
+    
+    /**
+     * Calcular ISR quincenal = ISR mensual / 2
+     */
+    public static function calculateBiweekly(float $monthlyIsr): float
+    {
+        return round($monthlyIsr / 2, 2);
+    }
+}