🎲 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/en.json

@@ -52,6 +52,17 @@
     "copyright": "© {year} Célian — Skycel",
     "built_with": "Built with Nuxt & Laravel"
   },
+  "consent": {
+    "aria_label": "Consent banner",
+    "message": "To save your adventure and let you pick up where you left off, I need your permission to store a few details on your device. Nothing personal, just your progress!",
+    "accept": "Sure, save my adventure",
+    "refuse": "No thanks, I prefer to stay anonymous"
+  },
+  "welcome_back": {
+    "message": "Welcome back, adventurer! You had started your exploration...",
+    "continue": "Continue",
+    "restart": "Start over"
+  },
   "pages": {
     "projects": {
       "title": "Projects",