Add per-assistant control for automatic default model mentions via a toggle switch in assistant settings. When disabled, users must manually select models via @ panel for each new topic instead of auto-populating from defaults.

- Add `enableDefaultModelMentions` field to Assistant type (defaults to true)
- Add inline Switch component in AssistantModelSettings next to "Default Models" label
- Update Inputbar logic to respect toggle setting:
  - Initial state checks toggle before seeding default models
  - Cache restoration respects toggle
  - Default sync respects toggle
  - New topic creation respects toggle when resetting mentions
- Add i18n translations for toggle label across all locales
- Backward compatible: undefined/null defaults to enabled behavior

Files modified:
- src/renderer/src/types/index.ts (line 51)
- src/renderer/src/pages/settings/AssistantSettings/AssistantModelSettings.tsx (lines 42-44, 250-260)
- src/renderer/src/pages/home/Inputbar/Inputbar.tsx (lines 116-118, 232-254, 463-485)
- src/renderer/src/i18n/locales/*.json (enableMentions key)