fix(audit): corrige errores criticos y mayores, mejora UX/accesibilidad y optimiza rendimiento

- Arregla @require_auth, permisos, race conditions, locks de caja/stock
- Elimina N+1 en layaway, flotilla, dashboard y global_invoice
- Asegura folios atomicos para CFDI, ordenes de servicio y polizas
- Protege client_secret de MercadoLibre en backend
- Conecta botones/filtros de config, customers, accounting e invoicing
- Mejora accesibilidad (labels/aria-label) y estados de carga/vacio
- Limpia accounting.js obsoleto y consolida accounting.v9.js
- Actualiza cache busting a v32 y Service Worker a v32
- Documenta todo en docs/AUDIT_Y_MEJORAS_2026-06-15.md

Tests: 35 passed

pos/static/css/accounting.css

@@ -1243,3 +1243,45 @@
       .summary-strip { grid-template-columns: 1fr; }
       .finance-grid { grid-template-columns: 1fr; }
     }
+
+    /* =========================================================================
+       MODAL DETALLE CUENTA POR COBRAR
+       ========================================================================= */
+    .modal-overlay {
+      position: fixed; inset: 0;
+      background: var(--overlay-backdrop);
+      display: flex; align-items: center; justify-content: center;
+      z-index: var(--z-modal);
+      opacity: 0; pointer-events: none;
+      transition: var(--transition-normal);
+    }
+    .modal-overlay.open { opacity: 1; pointer-events: auto; }
+
+    .modal-pago {
+      background: var(--color-bg-elevated); border: 1px solid var(--color-border);
+      border-radius: var(--radius-lg); box-shadow: var(--shadow-xl);
+      width: 560px; max-width: 95vw; max-height: 90vh; overflow-y: auto;
+      transform: translateY(20px); transition: var(--transition-normal);
+    }
+    .modal-overlay.open .modal-pago { transform: translateY(0); }
+
+    .modal-header {
+      display: flex; align-items: center; justify-content: space-between;
+      padding: var(--space-5) var(--space-6); border-bottom: 1px solid var(--color-border);
+    }
+    .modal-header h3 {
+      font-family: var(--font-heading); font-size: var(--text-h4);
+      font-weight: var(--heading-weight-primary); color: var(--color-text-primary);
+    }
+    .modal-close {
+      width: 36px; height: 36px; display: flex; align-items: center; justify-content: center;
+      background: transparent; border: 1px solid var(--color-border);
+      border-radius: var(--radius-md); cursor: pointer;
+      color: var(--color-text-muted); font-size: 18px; transition: var(--transition-fast);
+    }
+    .modal-close:hover { background: var(--color-surface-2); color: var(--color-text-primary); }
+
+    .modal-footer {
+      display: flex; align-items: center; justify-content: flex-end; gap: var(--space-3);
+      padding: var(--space-4) var(--space-6); border-top: 1px solid var(--color-border);
+    }