Fixed issue #5194 where changing the layout (List/Masonry) or sort direction
didn't update the UI until page refresh. The root cause was that ViewState
fields weren't marked as MobX observables, so the UI didn't react to changes
even though values were being persisted to localStorage.

Solution: Added constructor to ViewState that marks orderByTimeAsc and layout
fields as observable, following the same pattern used in other stores like
MemoFilterState.

Fixes #5194

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Co-Authored-By: Claude <noreply@anthropic.com>