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>

main.py

@@ -69,7 +69,7 @@ from app.modules.routes import (
     get_vendor_page_routes,
 )
 from app.utils.i18n import get_jinja2_globals
-from middleware.context import ContextMiddleware
+from middleware.frontend_type import FrontendTypeMiddleware
 from middleware.language import LanguageMiddleware
 from middleware.logging import LoggingMiddleware
 from middleware.theme_context import ThemeContextMiddleware
@@ -122,15 +122,15 @@ app.add_middleware(
 # Desired execution order:
 #   1. PlatformContextMiddleware  (detect platform from domain/path)
 #   2. VendorContextMiddleware    (detect vendor, uses platform_clean_path)
-#   3. ContextMiddleware          (detect context using clean_path)
-#   4. LanguageMiddleware         (detect language based on context)
+#   3. FrontendTypeMiddleware     (detect frontend type using FrontendDetector)
+#   4. LanguageMiddleware         (detect language based on frontend type)
 #   5. ThemeContextMiddleware     (load theme)
 #   6. LoggingMiddleware          (log all requests)
 #
 # Therefore we add them in REVERSE:
 #   - Add ThemeContextMiddleware FIRST (runs LAST in request)
 #   - Add LanguageMiddleware SECOND
-#   - Add ContextMiddleware THIRD
+#   - Add FrontendTypeMiddleware THIRD
 #   - Add VendorContextMiddleware FOURTH
 #   - Add PlatformContextMiddleware FIFTH
 #   - Add LoggingMiddleware LAST (runs FIRST for timing)
@@ -152,9 +152,9 @@ app.add_middleware(ThemeContextMiddleware)
 logger.info("Adding LanguageMiddleware (detects language based on context)")
 app.add_middleware(LanguageMiddleware)
 
-# Add context detection middleware (runs after vendor context extraction)
-logger.info("Adding ContextMiddleware (detects context type using clean_path)")
-app.add_middleware(ContextMiddleware)
+# Add frontend type detection middleware (runs after vendor context extraction)
+logger.info("Adding FrontendTypeMiddleware (detects frontend type using FrontendDetector)")
+app.add_middleware(FrontendTypeMiddleware)
 
 # Add vendor context middleware (runs after platform context)
 logger.info("Adding VendorContextMiddleware (detects vendor, uses platform_clean_path)")
@@ -170,7 +170,7 @@ logger.info("  Execution order (request →):")
 logger.info("    1. LoggingMiddleware (timing)")
 logger.info("    2. PlatformContextMiddleware (platform detection)")
 logger.info("    3. VendorContextMiddleware (vendor detection)")
-logger.info("    4. ContextMiddleware (context detection)")
+logger.info("    4. FrontendTypeMiddleware (frontend type detection)")
 logger.info("    5. LanguageMiddleware (language detection)")
 logger.info("    6. ThemeContextMiddleware (theme loading)")
 logger.info("    7. FastAPI Router")