fix: prioritize configured custom context

[baichuan335]

Summary

• Prefer the configured custom context in Cursor's context options and select it as the default model variant.
• Propagate the configured context token limit through the model catalog and keep it authoritative over Cursor-side context selections.
• Remove the (Custom) suffix and update the custom context guidance in both Chinese and English translations.
• Update the edited model in local state immediately after saving instead of waiting for a full application refresh.