Improves card dragging at the bottom of the playable area

[clpetersonucf]

Resolves #62, and by extension #55.

This was tricky since the arrangement of elements meant the desktop (which contains the draggable cards) and the dock (which contains the folders) occupy different stacking contexts, so z-index values were not shared. To function properly, the dock needed to remain "above" the desktop in z-height, but this caused it obfuscate cards behind it.

The solution I came up with was to move the dock element below the playable area, then reposition the folders by the inverse offset so they remain in their previous position. This allows the user to interact with folders normally, but the empty space around them is no longer blocking cards from being dragged.

This also applies overflow:hidden to the body element, which has the potential to resolve the behavior seen in #25 as well.

[jaadez]

Files "jump" back into play area and are still playable around folders and outer edges (around folders and corners).
LGTM 🙂👍

[dmols]

The cards no longer get stuck and unable to be picked up. I also didn't see the issue from #25 happen anymore, and the addition of resetting the card position works well! The area it needs to hit before a card gets reset seems a little early to me, but that might just be me nitpicking. Great work!