feat: multi-module improvements across merchant, store, i18n, and customer systems

- Fix platform-grouped merchant sidebar menu with core items at root level
- Add merchant store management (detail page, create store, team page)
- Fix store settings 500 error by removing dead stripe/API tab
- Move onboarding translations to module-owned locale files
- Fix onboarding banner i18n with server-side rendering + context inheritance
- Refactor login language selectors to use languageSelector() function (LANG-002)
- Move HTTPException handling to global exception handler in merchant routes (API-003)
- Add language selector to all login pages and portal headers
- Fix customer module: drop order stats from customer model, add to orders module
- Fix admin menu config visibility for super admin platform context
- Fix storefront auth and layout issues
- Add missing i18n translations for onboarding steps (en/fr/de/lb)

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

app/templates/shared/macros/feature_gate.html

@@ -355,13 +355,13 @@
         <div class="flex items-center justify-between">
             <div class="flex items-center gap-3">
                 <span x-html="$icon('rocket', 'w-5 h-5 text-purple-600 dark:text-purple-400')"></span>
-                <h3 class="text-lg font-semibold text-gray-900 dark:text-white" x-text="t('onboarding.banner.title')">Get Started</h3>
+                <h3 class="text-lg font-semibold text-gray-900 dark:text-white">{{ _("onboarding.banner.title") }}</h3>
                 <span class="text-sm text-gray-500 dark:text-gray-400"
                       x-text="`${completedSteps}/${totalSteps}`"></span>
             </div>
             <button @click="dismiss()"
                     class="text-gray-400 hover:text-gray-600 dark:hover:text-gray-300 transition-colors"
-                    :title="t('onboarding.banner.dismiss')">
+                    title="{{ _("onboarding.banner.dismiss") }}">
                 <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                     <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12"/>
                 </svg>