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>

app/modules/customers/routes/api/storefront.py

@@ -10,7 +10,7 @@ Public and authenticated endpoints for customer operations in storefront:
 Uses vendor from middleware context (VendorContextMiddleware).
 
 Implements dual token storage with path restriction:
-- Sets HTTP-only cookie with path=/shop (restricted to shop routes only)
+- Sets HTTP-only cookie with path=/storefront (restricted to storefront routes only)
 - Returns token in response for localStorage (API calls)
 """
 
@@ -182,14 +182,14 @@ def customer_login(
         else "unknown"
     )
 
-    cookie_path = "/shop"
+    cookie_path = "/storefront"
     if access_method == "path":
         full_prefix = (
             vendor_context.get("full_prefix", "/vendor/")
             if vendor_context
             else "/vendor/"
         )
-        cookie_path = f"{full_prefix}{vendor.subdomain}/shop"
+        cookie_path = f"{full_prefix}{vendor.subdomain}/storefront"
 
     response.set_cookie(
         key="customer_token",
@@ -240,14 +240,14 @@ def customer_logout(request: Request, response: Response):
         else "unknown"
     )
 
-    cookie_path = "/shop"
+    cookie_path = "/storefront"
     if access_method == "path" and vendor:
         full_prefix = (
             vendor_context.get("full_prefix", "/vendor/")
             if vendor_context
             else "/vendor/"
         )
-        cookie_path = f"{full_prefix}{vendor.subdomain}/shop"
+        cookie_path = f"{full_prefix}{vendor.subdomain}/storefront"
 
     response.delete_cookie(key="customer_token", path=cookie_path)