fix(i18n): Add missing 'welcome' translation key to en/es/fr.json

The HelpComprehensive.tsx uses introduction.welcome but the translation
files only had introduction.title. Added the welcome key to match
the German translation structure.

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Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/i18n/locales/en.json

@@ -2765,7 +2765,8 @@
       "usageQuota": "Usage & Quota"
     },
     "introduction": {
-      "title": "Welcome to SmoothSchedule",
+      "title": "Introduction",
+      "welcome": "Welcome to SmoothSchedule",
       "description": "SmoothSchedule is a complete scheduling platform designed to help businesses manage appointments, customers, staff, and services. This comprehensive guide covers everything you need to know to get the most out of the platform.",
       "tocHint": "Use the table of contents on the left to jump to specific sections, or scroll through the entire guide."
     },

frontend/src/i18n/locales/es.json

@@ -1665,7 +1665,8 @@
       "usageQuota": "Uso y Cuota"
     },
     "introduction": {
-      "title": "Bienvenido a SmoothSchedule",
+      "title": "Introducción",
+      "welcome": "Bienvenido a SmoothSchedule",
       "description": "SmoothSchedule es una plataforma completa de programación diseñada para ayudar a las empresas a gestionar citas, clientes, personal y servicios. Esta guía completa cubre todo lo que necesitas saber para aprovechar al máximo la plataforma.",
       "tocHint": "Usa la tabla de contenidos a la izquierda para saltar a secciones específicas, o desplázate por toda la guía."
     },

frontend/src/i18n/locales/fr.json

@@ -1605,7 +1605,8 @@
       "usageQuota": "Utilisation et Quota"
     },
     "introduction": {
-      "title": "Bienvenue sur SmoothSchedule",
+      "title": "Introduction",
+      "welcome": "Bienvenue sur SmoothSchedule",
       "description": "SmoothSchedule est une plateforme de planification complète conçue pour aider les entreprises à gérer les rendez-vous, clients, personnel et services. Ce guide complet couvre tout ce que vous devez savoir pour tirer le meilleur parti de la plateforme.",
       "tocHint": "Utilisez la table des matières à gauche pour accéder à des sections spécifiques, ou faites défiler l'ensemble du guide."
     },