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/templates/admin/partials/header.html

@@ -50,6 +50,44 @@
                 </button>
             </li>
 
+            <!-- Language selector -->
+            <li class="relative" x-data="languageSelector('{{ request.state.language|default('en') }}')">
+                <button
+                    @click="isLangOpen = !isLangOpen"
+                    @click.outside="isLangOpen = false"
+                    class="p-1 rounded-md focus:outline-none focus:shadow-outline-purple"
+                    aria-label="Change language"
+                >
+                    <span class="fi text-lg" :class="'fi-' + languageFlags[currentLang]"></span>
+                </button>
+                <div
+                    x-show="isLangOpen"
+                    x-cloak
+                    x-transition:enter="transition ease-out duration-100"
+                    x-transition:enter-start="transform opacity-0 scale-95"
+                    x-transition:enter-end="transform opacity-100 scale-100"
+                    x-transition:leave="transition ease-in duration-75"
+                    x-transition:leave-start="transform opacity-100 scale-100"
+                    x-transition:leave-end="transform opacity-0 scale-95"
+                    class="absolute right-0 w-44 mt-2 bg-white dark:bg-gray-700 rounded-lg shadow-lg border border-gray-100 dark:border-gray-600 py-1 z-50"
+                >
+                    <template x-for="lang in languages" :key="lang">
+                        <button
+                            @click="setLanguage(lang)"
+                            type="button"
+                            class="flex items-center gap-3 w-full px-4 py-2 text-sm font-medium transition-colors"
+                            :class="currentLang === lang
+                                ? 'bg-purple-50 dark:bg-purple-900/20 text-purple-700 dark:text-purple-300'
+                                : 'text-gray-700 dark:text-gray-300 hover:bg-gray-100 dark:hover:bg-gray-600'"
+                        >
+                            <span class="fi" :class="'fi-' + languageFlags[lang]"></span>
+                            <span x-text="languageNames[lang]"></span>
+                            <span x-show="currentLang === lang" x-html="$icon('check', 'w-4 h-4 ml-auto')" class="text-purple-600 dark:text-purple-400"></span>
+                        </button>
+                    </template>
+                </div>
+            </li>
+
             <!-- Messages link with badge -->
             <li class="relative" x-data="headerMessages()">
                 <a href="/admin/messages"