Fix broken logger format string and leaked exception in config save error

- pages_v3.py: plain string was used instead of %-style substitution,
  so every manifest-read failure logged the literal "{plugin_id}"
- api_v3.py save_main_config: exception message was still leaking
  through the error response; replace with generic message (consistent
  with the rest of the CodeQL sweep in this PR)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/blueprints/api_v3.py

@@ -1039,12 +1039,10 @@ def save_main_config():
 
         return success_response(message='Configuration saved successfully')
     except Exception as e:
-        import logging
         logger.error("Error saving config", exc_info=True)
         return error_response(
             ErrorCode.CONFIG_SAVE_FAILED,
-            f"Error saving configuration: {e}",
-
+            "An error occurred; see logs for details",
             status_code=500
         )