Preserve common config during proxy takeover

Update takeover backup generation to rebuild effective provider settings with common config applied before saving restore snapshots.

Keep Codex mcp_servers entries when hot-switching providers under takeover so restore does not drop live-only MCP config.

Migrate legacy providers with inferred common-config usage to explicit commonConfigEnabled=true markers during startup and default imports, and cover the new behavior with proxy and provider regression tests.

src-tauri/src/commands/provider.rs

@@ -117,6 +117,8 @@ fn import_default_config_internal(state: &AppState, app_type: AppType) -> Result
                 _ => {}
             }
         }
+
+        ProviderService::migrate_legacy_common_config_usage_if_needed(state, app_type.clone())?;
     }
 
     Ok(imported)