Feat: Add gap in planner when one or more days were skipped

[crosspolar]

Indicate that in your planner a day or more were skipped by adding a small gap, here tuesday is without plan:

I can use it since in one household for a group of children I only plan from monday to friday, so it helps me separating the weeks :)

[TomBursch]

Sorry, I've only looked at this now. The main problem with this is that the cards are spaced and sized exactly such that there always fits the same number of them in a row. With this PR on mobile, the cards wrap early and leave a large gap on the right side.

For example, on a screen that would have a grid width of four:

I like the idea in general, but maybe a divider or something else that increases the spacing uniformly would be better?