🎲 Add Pinia progression store & GDPR consent banner (Story 1.6)

Implements useProgressionStore with conditional localStorage persistence
(only after RGPD consent), immersive ConsentBanner with narrator style,
WelcomeBack component for returning visitors, and connects progress bar
in header to store.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/i18n/fr.json

@@ -52,6 +52,17 @@
     "copyright": "© {year} Célian — Skycel",
     "built_with": "Construit avec Nuxt & Laravel"
   },
+  "consent": {
+    "aria_label": "Bandeau de consentement",
+    "message": "Pour m\u00e9moriser ton aventure et te permettre de la reprendre plus tard, j'ai besoin de ton accord pour stocker quelques informations sur ton appareil. Rien de personnel, juste ta progression !",
+    "accept": "D'accord, m\u00e9morise mon aventure",
+    "refuse": "Non merci, je pr\u00e9f\u00e8re rester anonyme"
+  },
+  "welcome_back": {
+    "message": "Content de te revoir, aventurier ! Tu avais commenc\u00e9 ton exploration...",
+    "continue": "Reprendre",
+    "restart": "Recommencer"
+  },
   "pages": {
     "projects": {
       "title": "Projets",