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/platform/base.html

@@ -86,10 +86,11 @@
                             @click="isOpen = !isOpen"
                             @click.outside="isOpen = false"
                             type="button"
-                            class="p-2 text-gray-600 dark:text-gray-400 hover:text-gray-900 dark:hover:text-white hover:bg-gray-100 dark:hover:bg-gray-700 rounded-lg transition-colors"
+                            class="inline-flex items-center gap-1 p-2 text-gray-600 dark:text-gray-400 hover:text-gray-900 dark:hover:text-white hover:bg-gray-100 dark:hover:bg-gray-700 rounded-lg transition-colors"
                             :class="{ 'bg-gray-100 dark:bg-gray-700': isOpen }"
                         >
-                            <span class="fi text-lg" :class="'fi-' + flags[currentLang]"></span>
+                            <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="M21 12a9 9 0 01-9 9m9-9a9 9 0 00-9-9m9 9H3m9 9a9 9 0 01-9-9m9 9c1.657 0 3-4.03 3-9s-1.343-9-3-9m0 18c-1.657 0-3-4.03-3-9s1.343-9 3-9m-9 9a9 9 0 019-9"/></svg>
+                            <span class="text-xs font-semibold uppercase" x-text="currentLang"></span>
                         </button>
                         <div
                             x-show="isOpen"