Keep synthesis disabled for new profiles

[calvintvu]

Context

New profiles should not automatically enable synthesis merely because a supported backend is detected. Keep the available backend and model preselected so users can opt in without extra setup.

Changes

• Leave synthesis disabled when initializing unpersisted settings.
• Preserve automatic selection of an available backend and its default model.
• Add regression coverage for new-profile initialization.

Test

• npm test — passed, 80 tests
• npm run build — passed
• npm run lint — passed with existing warnings
• npm run format:check — passed
• git diff --check — passed

Screenshots

• Fresh profile settings — AI synthesis is disabled while a backend and model remain preselected