[6.x] Fix calendar view for date fields using a format without time

[duncanmcclean]

This pull request fixes an issue where the calendar view for collections would break when the collection's date field uses a format without a time component, like Y-m-d. The calendar would render for a moment, then everything apart from the weekday header row would disappear (or, depending on the build, entries would simply never show).

This was happening because when a date field's format has no time, preProcessIndex returns a plain Y-m-d string rather than a full ISO 8601 datetime string. The calendar passed that straight into parseAbsoluteToLocal, which throws for date-only strings, breaking the render.

This PR fixes it by adding a getEntryDate helper which parses date-only values with parseDate and datetime values with parseAbsoluteToLocal. In the week view, date-only entries are shown against midnight. This PR also hides the time label on month view entries when the date format has no time, since it would otherwise show a meaningless, timezone-shifted midnight.

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Fixes #15245