Default assistant and CherryAI defaults are seeded

What changed

Fresh v2 databases seed a persisted default assistant backed by the managed CherryAI cherryai::qwen model. The seeded assistant name is chosen once from Electron's preferred system languages: Chinese languages (zh-*) use Cherry 助手, and all other languages use Cherry Assistant.

The CherryAI default seeder also inserts missing default-model preference rows for:

• chat.default_model_id
• topic.naming.model_id
• feature.quick_assistant.model_id
• feature.translate.model_id

Existing preference rows are preserved, including intentional null values such as translate's "follow the default model" state.

Topic auto-naming now uses topic.naming.model_id instead of the current assistant's model. If that preference is invalid or points to a missing model, topic naming falls back to the managed CherryAI default model.

Settings pickers that still have the legacy renderer default-assistant sentinel now prefer the persisted seeded default assistant when it exists, so fresh installs do not show duplicate default assistant choices.

The managed CherryAI default model is internal app bootstrap data. It is not listed by the API gateway /v1/models endpoint and cannot be invoked through gateway chat/message routes.

Why this matters to the user

Fresh profiles whose preferred system language is Chinese start with Cherry 助手; other fresh profiles start with Cherry Assistant. The seeded assistant remains ordinary user data and can be renamed or deleted, and it is not automatically renamed if the app language changes later.

Existing v2 profiles that are missing one of the default-model preference rows may receive cherryai::qwen for that missing row the next time the seeder runs. Existing non-empty values and existing null values are not overwritten.

Local API clients should not rely on the CherryAI managed default model as a gateway-accessible model. Select an explicitly configured user/provider model for gateway traffic.

What the user should do

Nothing — automatic. Rename the default assistant manually if a different localized or custom name is preferred.