feat(ux): first-time onboarding overlay for new users

Shows a 3-screen modal overlay on first dashboard visit explaining key
features; dismissed state is persisted to localStorage so it never
reappears.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/locales/fr.json

@@ -780,5 +780,16 @@
     "notificationEnabled": "Notifications actives",
     "notificationDenied": "Notifications bloquées",
     "notificationHint": "Recevez des notifications même lorsque l'application est ouverte."
+  },
+  "onboarding": {
+    "step1Title": "Welcome to Oikos",
+    "step1Body": "Your personal family planner. Tasks, calendar, shopping and more – all in one place.",
+    "step2Title": "Everything at a glance",
+    "step2Body": "Use the navigation below to reach all modules. The + button creates new entries quickly.",
+    "step3Title": "Ready to go",
+    "step3Body": "The dashboard shows you the most important information at a glance. Customize it under \"Customize\".",
+    "next": "Next",
+    "done": "Get started",
+    "skip": "Skip"
   }
 }