mecatui: add form-scoped assisted schedule timing #2415

Outcome

Offer optional model-assisted timing interpretation inside the mecatui schedule form. PR #2398 supports finite phrases and cron; ordinary chat already manages schedules conversationally.

Scope

• Convert timing intent into existing cron or one-shot fields and an explicit timezone.
• Show canonical trigger, timezone, next occurrence, and ambiguity for review before creation.
• Keep direct editing and explicit Create; assistance must not implicitly create a schedule or send a normal-chat prompt.
• Define model cost/permissions and cancellation; unavailable assistance, ambiguity, or invalid proposals preserve the draft with actionable feedback.
• Offline tests cover confirmation, failures, and stale interpretation results.

Related: #2398. No automatic form model call is included in that PR.