- Add ProviderModelSelect to heartbeat config dialog (allowEmpty, verify button)
- Backend: accept providerName in HEARTBEAT.SET, resolve to UUID via GetProviderByName
- Add ModelOverride to RunRequest, used by Loop when set (cheaper model for heartbeat)
- Ticker passes heartbeat model override to agent RunRequest
- Fix: InvalidateCache after UpdateState so ListDue picks up new next_run_at immediately
- i18n: add sectionModel/modelHint keys (en/vi/zh)