fix: language switcher stuck in French on store dashboard

Three compounding bugs prevented language switching on the store dashboard:
- Cookie missing path="/", scoping it to the API endpoint path only
- STORE frontend resolution chain ignored the cookie entirely
- Store header used inline x-data with wrong language names instead of shared languageSelector()

Also updates architecture doc with correct per-frontend resolution priorities,
cookie name, API endpoint path, and file references.

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

app/templates/store/partials/header.html

@@ -37,33 +37,7 @@
             </li>
 
             <!-- Language selector -->
-            <li class="relative" x-data="{
-                isLangOpen: false,
-                currentLang: '{{ request.state.language|default("fr") }}',
-                languages: ['en', 'fr', 'de', 'lb'],
-                languageNames: { 'en': 'English', 'fr': 'Francais', 'de': 'Deutsch', 'lb': 'Letzebuerg' },
-                languageFlags: { 'en': 'gb', 'fr': 'fr', 'de': 'de', 'lb': 'lu' },
-                async setLanguage(lang) {
-                    if (lang === this.currentLang) {
-                        this.isLangOpen = false;
-                        return;
-                    }
-                    try {
-                        const response = await fetch('/api/v1/platform/language/set', {
-                            method: 'POST',
-                            headers: { 'Content-Type': 'application/json' },
-                            body: JSON.stringify({ language: lang })
-                        });
-                        if (response.ok) {
-                            this.currentLang = lang;
-                            window.location.reload();
-                        }
-                    } catch (error) {
-                        console.error('Failed to set language:', error);
-                    }
-                    this.isLangOpen = false;
-                }
-            }">
+            <li class="relative" x-data="languageSelector('{{ request.state.language|default('fr') }}')">
                 <button
                     @click="isLangOpen = !isLangOpen"
                     @click.outside="isLangOpen = false"
@@ -179,4 +153,4 @@
             </li>
         </ul>
     </div>
-</header>
+</header>

app/utils/i18n.py

@@ -351,15 +351,20 @@ def get_jinja2_globals(language: str = None) -> dict:
 def resolve_store_dashboard_language(
     user_preferred: str | None,
     store_dashboard: str | None,
+    cookie_language: str | None = None,
 ) -> str:
     """
     Resolve language for store dashboard.
 
     Priority:
-    1. User's preferred_language (if set)
-    2. Store's dashboard_language
-    3. System default (fr)
+    1. Cookie (explicit UI action via language switcher)
+    2. User's preferred_language (DB preference from profile settings)
+    3. Store's dashboard_language
+    4. System default (fr)
     """
+    if cookie_language and cookie_language in SUPPORTED_LANGUAGES:
+        return cookie_language
+
     if user_preferred and user_preferred in SUPPORTED_LANGUAGES:
         return user_preferred