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/utils/i18n.py

@@ -398,22 +398,24 @@ def resolve_storefront_language(
         browser_language: Primary language from Accept-Language header
         enabled_languages: List of languages enabled for this storefront
     """
-    candidates = [
-        customer_preferred,
-        session_language,
-        store_storefront,
-        browser_language,
-        DEFAULT_LANGUAGE,
-    ]
+    # Explicit user choices (customer preference, session/cookie) should be
+    # respected if they are valid platform languages, regardless of
+    # store's enabled_languages. The store setting only constrains automatic
+    # fallback (browser language, store default).
+    explicit_choices = [customer_preferred, session_language]
+    for lang in explicit_choices:
+        if lang and lang in SUPPORTED_LANGUAGES:
+            return lang
 
-    # Filter to enabled languages if specified
+    # For automatic fallback, filter by store's enabled languages
+    fallback_candidates = [store_storefront, browser_language, DEFAULT_LANGUAGE]
     if enabled_languages:
         enabled_set = set(enabled_languages)
-        for lang in candidates:
+        for lang in fallback_candidates:
             if lang and lang in SUPPORTED_LANGUAGES and lang in enabled_set:
                 return lang
     else:
-        for lang in candidates:
+        for lang in fallback_candidates:
             if lang and lang in SUPPORTED_LANGUAGES:
                 return lang