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>

middleware/language.py

@@ -38,10 +38,11 @@ class LanguageMiddleware(BaseHTTPMiddleware):
     Middleware to detect and set the request language.
 
     Sets request.state.language based on context:
-    - Admin: Always English (for now)
-    - Store dashboard: User preference → Store dashboard_language → default
+    - Admin: Cookie → User preference → English
+    - Merchant: Cookie → User preference → Platform default
+    - Store dashboard: Cookie → User preference → Store dashboard_language → default
     - Storefront: Customer preference → Cookie → Store storefront_language → browser → default
-    - API: Accept-Language header → default
+    - Platform: Cookie → Browser → Platform default
     """
 
     async def dispatch(self, request: Request, call_next) -> Response:
@@ -61,9 +62,9 @@ class LanguageMiddleware(BaseHTTPMiddleware):
 
         # Resolve language based on frontend type
         if frontend_type == FrontendType.ADMIN:
-            # Admin dashboard: respect user's preferred language
+            # Admin dashboard: cookie → user preference → English default
             user_preferred = self._get_user_language_from_token(request)
-            language = user_preferred or "en"
+            language = cookie_language or user_preferred or "en"
 
         elif frontend_type == FrontendType.STORE:
             # Store dashboard
@@ -90,6 +91,11 @@ class LanguageMiddleware(BaseHTTPMiddleware):
                 enabled_languages=enabled_languages,
             )
 
+        elif frontend_type == FrontendType.MERCHANT:
+            # Merchant portal: cookie → user preference → platform default
+            user_preferred = self._get_user_language_from_token(request)
+            language = cookie_language or user_preferred or DEFAULT_LANGUAGE
+
         elif frontend_type == FrontendType.PLATFORM:
             # Platform marketing pages: Use cookie, browser, or default
             language = cookie_language or browser_language or DEFAULT_LANGUAGE