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/storefront/base.html

@@ -150,10 +150,11 @@
                         <button
                             @click="isLangOpen = !isLangOpen"
                             @click.outside="isLangOpen = false"
-                            class="p-2 rounded-lg hover:bg-gray-100 dark:hover:bg-gray-700"
+                            class="inline-flex items-center gap-1 p-2 rounded-lg hover:bg-gray-100 dark:hover:bg-gray-700"
                             aria-label="Change language"
                         >
-                            <span class="fi text-lg" :class="'fi-' + languageFlags[currentLang]"></span>
+                            <span x-html="$icon('globe-alt', 'w-5 h-5')"></span>
+                            <span class="text-xs font-semibold uppercase" x-text="currentLang"></span>
                         </button>
                         <div
                             x-show="isLangOpen"