mecatui: remember the schedule creator timezone preference #2418

Outcome

Remember an explicitly chosen timezone as the next new schedule form's default. PR #2398 visibly defaults to UTC and deliberately does not infer or remember a timezone.

Scope

• Define client preference ownership and scope using existing local preference storage.
• Persist only the accepted IANA identifier, not an unfinished search query.
• Show the effective default and allow explicit override before Create.
• Invalid/unavailable saved preferences produce clear correction behavior; never silently reinterpret a stored trigger or rewrite existing schedules.
• Offline tests cover reload, invalid preference, and explicit override.

Related: #2398. This is a new-form preference, not geolocation or host-timezone inference.