fix(security): correct error inference, remove debug log leak, consolidate config handlers

- _infer_error_code: map Config* exceptions to CONFIG_LOAD_FAILED
  (ConfigError is only raised by load_config(), so CONFIG_SAVE_FAILED
  produced wrong safe message and wrong suggested_fixes)
- Remove leftover DEBUG logs in save_main_config that dumped full
  request body and all HTTP headers (Authorization, Cookie, etc.)
- Replace dead FileNotFoundError/JSONDecodeError/IOError handlers in
  get_dim_schedule_config with single ConfigError catch (load_config
  already wraps these into ConfigError)
- Remove redundant local `from src.exceptions import ConfigError`
  imports now covered by top-level import
- Strip str(e) from client-facing error messages in dim schedule handler

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

src/web_interface/errors.py

@@ -254,7 +254,7 @@ class WebInterfaceError:
         exception_name = type(exception).__name__
         
         if "Config" in exception_name:
-            return ErrorCode.CONFIG_SAVE_FAILED
+            return ErrorCode.CONFIG_LOAD_FAILED
         elif "Plugin" in exception_name:
             return ErrorCode.PLUGIN_LOAD_FAILED
         elif "Permission" in exception_name or "Access" in exception_name: