WIP: add image model type support

[sheepbox8646]

Summary

• add a first-class image model type with generate and edit compatibilities across the schema, backend model layer, and generated SDK
• let generate_image use dedicated Twilight Images API models while keeping existing chat models with image-output selectable
• update provider import, model management, and bot settings UI so image tools can pick either compatible chat models or dedicated image models

Test plan

• go test ./internal/models ./internal/agent/tools ./internal/handlers
• pre-commit checks during git commit (Go lint/test and frontend ESLint)
• mise run build-unified
• pnpm exec tsc --noEmit still reports existing packages/ui .vue module resolution errors unrelated to this change