fix: resolve all architecture validation warnings

Service layer:
- Remove db.commit() calls from credentials_service.py (SVC-006)
- Move transaction control to API endpoint level
- Rename client.py -> client_service.py (NAM-002)
- Rename credentials.py -> credentials_service.py (NAM-002)

JavaScript:
- Use centralized logger in admin letzshop.js (JS-001)
- Replace console.log/error with LogConfig logger

Frontend templates:
- Use page_header_flex macro for page header (FE-007)
- Use error_state macro for error display (FE-003)
- Use table_wrapper macro for vendors table (FE-005)
- Use modal macro for configuration and orders modals (FE-004)

All 31 Letzshop tests pass. Architecture validation: 0 errors, 0 warnings.

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

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

app/api/v1/admin/letzshop.py

@@ -164,6 +164,7 @@ def create_or_update_vendor_credentials(
         auto_sync_enabled=credentials_data.auto_sync_enabled,
         sync_interval_minutes=credentials_data.sync_interval_minutes,
     )
+    db.commit()
 
     logger.info(
         f"Admin {current_admin.email} updated Letzshop credentials for vendor {vendor.name}"
@@ -211,6 +212,7 @@ def update_vendor_credentials(
             auto_sync_enabled=credentials_data.auto_sync_enabled,
             sync_interval_minutes=credentials_data.sync_interval_minutes,
         )
+        db.commit()
     except CredentialsNotFoundError:
         raise ResourceNotFoundException(
             "LetzshopCredentials", str(vendor_id),
@@ -256,6 +258,7 @@ def delete_vendor_credentials(
             "LetzshopCredentials", str(vendor_id),
             message=f"Letzshop credentials not configured for vendor {vendor.name}"
         )
+    db.commit()
 
     logger.info(
         f"Admin {current_admin.email} deleted Letzshop credentials for vendor {vendor.name}"