fix(config): drop retired skin/skin_options keys instead of validating them

A config.json written while the skin system existed can carry skin and
skin_options in any plugin section, and most plugin schemas set
additionalProperties: false. They are no longer core plugin properties;
RETIRED_PLUGIN_KEYS in schema_manager lists them and
drop_retired_plugin_keys removes them (unless the plugin's own schema
declares the name) in prepare_plugin_config, which loading, hot reload,
GET /plugins/config and both web saves already share, and in
validate_config_against_schema for callers that validate a raw section.
POST /plugins/config and /config/main also drop them from the stored
section they merge into, so they leave config.json on the next save.

Tests cover the load path (real PluginManager.load_plugin: no schema
warning, not degraded), raw and prepared validation,
validate_all_plugin_configs, and the JSON, form and /config/main saves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/config.py

@@ -1052,10 +1052,14 @@ def save_main_config():
                 if error:
                     return error
 
-                # Deep merge regular config into main config
+                # Deep merge regular config into main config, dropping
+                # retired core keys (skin, skin_options) from the stored section
+                from src.plugin_system.schema_manager import drop_retired_plugin_keys
                 stored_section = current_config.get(plugin_id)
                 current_config[plugin_id] = deep_merge(
-                    stored_section if isinstance(stored_section, dict) else {}, regular_config)
+                    drop_retired_plugin_keys(
+                        stored_section if isinstance(stored_section, dict) else {}, schema),
+                    regular_config)
                 if secrets_config:
                     plugin_secrets_updates[plugin_id] = secrets_config