feat: add language switching to admin and merchant frontends

- Add cookie to ADMIN resolution chain (cookie → user_pref → "en")
- Add explicit MERCHANT resolution (cookie → user_pref → "fr")
- Add language selector dropdown to admin and merchant headers
- Add languageSelector() function to merchant init-alpine.js
- Add flag-icons CSS and i18n.js setup to merchant base template
- Add compact flag-based language selector to both login pages
- Make lang attribute dynamic on all base and login templates
- Pass current_language to login route template context
- Update architecture doc with ADMIN/MERCHANT resolution priorities

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

app/modules/core/static/merchant/js/init-alpine.js

@@ -201,3 +201,49 @@ function data() {
         }
     };
 }
+
+/**
+ * Language Selector Component
+ * Alpine.js component for language switching in merchant portal
+ */
+function languageSelector(currentLang, enabledLanguages) {
+    return {
+        isLangOpen: false,
+        currentLang: currentLang || 'fr',
+        languages: enabledLanguages || ['en', 'fr', 'de', 'lb'],
+        languageNames: {
+            'en': 'English',
+            'fr': 'Français',
+            'de': 'Deutsch',
+            'lb': 'Lëtzebuergesch'
+        },
+        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;
+        }
+    };
+}
+
+window.languageSelector = languageSelector;