docs: update authentication dependency names across documentation

Updated all documentation to use correct authentication dependency names:
- HTML pages: get_current_admin_from_cookie_or_header, get_current_vendor_from_cookie_or_header, get_current_customer_from_cookie_or_header
- API endpoints: get_current_admin_api, get_current_vendor_api, get_current_customer_api

Changes:
- Updated authentication flow diagrams with correct dependency names for admin and vendor flows
- Added comprehensive customer/shop authentication flow documentation
- Updated cookie isolation architecture to show all three contexts (admin, vendor, shop)
- Expanded security boundary enforcement diagram to include shop routes
- Added customer cross-context prevention examples
- Updated all code examples in frontend and backend documentation
- Fixed import statements to use app.api.deps instead of app.core.auth

Files updated:
- docs/api/authentication-flow-diagrams.md (added customer flows)
- docs/frontend/admin/page-templates.md
- docs/frontend/admin/architecture.md
- docs/frontend/shared/ui-components.md
- docs/frontend/shared/sidebar.md
- docs/development/exception-handling.md
- docs/architecture/diagrams/vendor-domain-diagrams.md
- docs/backend/admin-integration-guide.md
- docs/backend/admin-feature-integration.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

docs/development/exception-handling.md

@@ -195,7 +195,7 @@ Endpoints rely on service layer exceptions and don't handle HTTPException:
 def toggle_user_status(
     user_id: int,
     db: Session = Depends(get_db),
-    current_admin: User = Depends(get_current_admin_user),  # May raise AdminRequiredException
+    current_admin: User = Depends(get_current_admin_api),  # May raise AdminRequiredException
 ):
     # Service raises UserNotFoundException, CannotModifySelfException, etc.
     user, message = admin_service.toggle_user_status(db, user_id, current_admin.id)