fix: resolve settings page icon and 404 errors

- Change icon from 'envelope' to 'mail' (envelope not in icons.js)
- Add default query param to GET /admin/settings/{key} endpoint
- Return AdminSettingDefaultResponse instead of 404 when default provided
- Update loadShippingSettings() to use default param for carrier settings

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

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

app/api/v1/admin/settings.py

@@ -24,6 +24,7 @@ from app.services.admin_settings_service import admin_settings_service
 from models.database.user import User
 from models.schema.admin import (
     AdminSettingCreate,
+    AdminSettingDefaultResponse,
     AdminSettingListResponse,
     AdminSettingResponse,
     AdminSettingUpdate,
@@ -75,16 +76,24 @@ def get_setting_categories(
     }
 
 
-@router.get("/{key}", response_model=AdminSettingResponse)
+@router.get("/{key}", response_model=AdminSettingResponse | AdminSettingDefaultResponse)
 def get_setting(
     key: str,
+    default: str | None = Query(None, description="Default value if setting not found"),
     db: Session = Depends(get_db),
     current_admin: User = Depends(get_current_admin_api),
-):
-    """Get specific setting by key."""
+) -> AdminSettingResponse | AdminSettingDefaultResponse:
+    """Get specific setting by key.
+
+    If `default` is provided and the setting doesn't exist, returns a response
+    with the default value instead of 404.
+    """
     setting = admin_settings_service.get_setting_by_key(db, key)
 
     if not setting:
+        if default is not None:
+            # Return default value without creating the setting
+            return AdminSettingDefaultResponse(key=key, value=default, exists=False)
         raise ResourceNotFoundException(resource_type="Setting", identifier=key)
 
     return AdminSettingResponse.model_validate(setting)