No keyboard navigation except Enter to open day

I have searched open and closed issues for duplicates.

## Missing Feature
The keyboard cursor keys have no effect on the open indicator. Nor the Tab or Page keys. The only keys with effect are `Enter` to open the selected day and `Esc` to close the popover.

**Describe the solution you'd like**
Cursor keys should change the selected day. (Shift)Tab key or Page keys should change the month.

**Existing work**
The Calendar app implements cursor key navigation but not a change month shortcut afaict.

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