🎲 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/components/layout/AppHeader.vue

@@ -22,9 +22,12 @@
       </nav>
 
       <div class="flex items-center gap-3">
-        <!-- Placeholder barre progression (Epic 3) -->
+        <!-- Barre de progression -->
         <div class="hidden md:block w-24 h-1.5 bg-sky-text/10 rounded-full overflow-hidden">
-          <div class="h-full bg-sky-accent/40 rounded-full" style="width: 0%"></div>
+          <div
+            class="h-full bg-sky-accent/40 rounded-full transition-all duration-500"
+            :style="{ width: progressPercent + '%' }"
+          />
         </div>
 
         <UiLanguageSwitcher />
@@ -78,7 +81,12 @@
 </template>
 
 <script setup lang="ts">
+import { useProgressionStore } from '~/stores/progression'
+
 const localePath = useLocalePath()
+const store = useProgressionStore()
+
+const progressPercent = computed(() => store.progressPercent)
 
 const mobileOpen = ref(false)
 const scrolled = ref(false)