🎲 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/app/layouts/default.vue

@@ -7,5 +7,9 @@
     </main>
 
     <LayoutAppFooter />
+
+    <ClientOnly>
+      <LayoutConsentBanner />
+    </ClientOnly>
   </div>
 </template>