Improve MK2 calendar generation UX

[LennartvdM]

Summary

• add a calendar generation loading state that clears the current visualization, shows feedback, and disables the trigger button while a run is in-flight
• surface the active run number/timestamp in the visualization pane and keep history focus cleared until a new result arrives
• style the new loading overlay and run metadata elements for the calendar visuals panel

Testing

• Not run (UI-only changes)

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Codex Task

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