refactor: centralize frontend detection with FrontendDetector

Major architecture change to unify frontend detection:

## Problem Solved
- Eliminated code duplication across 3 middleware files
- Fixed incomplete path detection (now detects /api/v1/admin/*)
- Unified on FrontendType enum (deprecates RequestContext)
- Added request.state.frontend_type for all requests

## New Components
- app/core/frontend_detector.py: Centralized FrontendDetector class
- middleware/frontend_type.py: FrontendTypeMiddleware (replaces ContextMiddleware)
- docs/architecture/frontend-detection.md: Complete architecture documentation

## Changes
- main.py: Use FrontendTypeMiddleware instead of ContextMiddleware
- middleware/context.py: Deprecated (kept for backwards compatibility)
- middleware/platform_context.py: Use FrontendDetector.is_admin()
- middleware/vendor_context.py: Use FrontendDetector.is_admin()
- middleware/language.py: Use FrontendType instead of context_value
- app/exceptions/handler.py: Use FrontendType.STOREFRONT
- app/exceptions/error_renderer.py: Use FrontendType
- Customer routes: Cookie path changed from /shop to /storefront

## Documentation
- docs/architecture/frontend-detection.md: New comprehensive docs
- docs/architecture/middleware.md: Updated for new system
- docs/architecture/request-flow.md: Updated for FrontendType
- docs/backend/middleware-reference.md: Updated API reference

## Tests
- tests/unit/core/test_frontend_detector.py: 37 new tests
- tests/unit/middleware/test_frontend_type.py: 11 new tests
- tests/unit/middleware/test_context.py: Updated for compatibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

middleware/language.py

@@ -18,6 +18,7 @@ from starlette.middleware.base import BaseHTTPMiddleware
 from starlette.requests import Request
 from starlette.responses import Response
 
+from app.modules.enums import FrontendType
 from app.utils.i18n import (
     DEFAULT_LANGUAGE,
     SUPPORTED_LANGUAGES,
@@ -45,9 +46,8 @@ class LanguageMiddleware(BaseHTTPMiddleware):
 
     async def dispatch(self, request: Request, call_next) -> Response:
         """Process the request and set language."""
-        # Get context type from previous middleware
-        context_type = getattr(request.state, "context_type", None)
-        context_value = context_type.value if context_type else None
+        # Get frontend type from FrontendTypeMiddleware
+        frontend_type = getattr(request.state, "frontend_type", None)
 
         # Get vendor from previous middleware (if available)
         vendor = getattr(request.state, "vendor", None)
@@ -59,13 +59,13 @@ class LanguageMiddleware(BaseHTTPMiddleware):
         accept_language = request.headers.get("accept-language")
         browser_language = parse_accept_language(accept_language)
 
-        # Resolve language based on context
-        if context_value == "admin":
+        # Resolve language based on frontend type
+        if frontend_type == FrontendType.ADMIN:
             # Admin dashboard: English only (for now)
             # TODO: Implement admin language support later
             language = "en"
 
-        elif context_value == "vendor_dashboard":
+        elif frontend_type == FrontendType.VENDOR:
             # Vendor dashboard
             user_preferred = self._get_user_language_from_token(request)
             vendor_dashboard = vendor.dashboard_language if vendor else None
@@ -75,7 +75,7 @@ class LanguageMiddleware(BaseHTTPMiddleware):
                 vendor_dashboard=vendor_dashboard,
             )
 
-        elif context_value == "shop":
+        elif frontend_type == FrontendType.STOREFRONT:
             # Storefront
             customer_preferred = self._get_customer_language_from_token(request)
             vendor_storefront = vendor.storefront_language if vendor else None
@@ -89,12 +89,12 @@ class LanguageMiddleware(BaseHTTPMiddleware):
                 enabled_languages=enabled_languages,
             )
 
-        elif context_value == "api":
-            # API requests: Use Accept-Language or cookie
+        elif frontend_type == FrontendType.PLATFORM:
+            # Platform marketing pages: Use cookie, browser, or default
             language = cookie_language or browser_language or DEFAULT_LANGUAGE
 
         else:
-            # Fallback: Use cookie, browser, or default
+            # Fallback (API or unknown): Use Accept-Language or cookie
             language = cookie_language or browser_language or DEFAULT_LANGUAGE
 
         # Validate language is supported
@@ -109,13 +109,14 @@ class LanguageMiddleware(BaseHTTPMiddleware):
             "code": language,
             "cookie": cookie_language,
             "browser": browser_language,
-            "context": context_value,
+            "frontend_type": frontend_type.value if frontend_type else None,
         }
 
         # Log language detection for debugging
+        frontend_value = frontend_type.value if frontend_type else "unknown"
         logger.debug(
             f"Language detected: {language} "
-            f"(context={context_value}, cookie={cookie_language}, browser={browser_language})"
+            f"(frontend={frontend_value}, cookie={cookie_language}, browser={browser_language})"
         )
 
         # Process request