Improve UX when dragging items over inventory arrows

Inventory scrolls when dragging items over inventory arrows (to make it possible to use items on items that aren't on the same "page"). This works a bit iffy. For example:
* it seems to depend on the leftmost visible inventory item how hard it is to "hit" the left arrow with a dragged item
* in the first locker room, the trashcan at the bottom left corner of the room may interfere with "hitting" the left arrow

Also previously observed: trying to drag past the leftmost item on top of the left arrow, seems to leave the leftmost item glow on. Not sure this still happens (as of October 2025).

Some ideas:

* Bigger inventory arrows
* Intersection checking priority in dragmove event handling? (inventory arrows are last, maybe could be first?)
* Glow on inventory arrows
* Don't glow on the item next to arrow (obviously)
* Objects in rooms have .cache() and .drawHitFromCache(); not honestly sure what that does, but maybe it might help?

I'm also open to a redesign of the inventory UX , for example:
* rendering the full inventory but only having part of it visible
* scrolling it freely with touch drag
* able to reorder inventory items with drag and drop

Start by tracing the inventory dragmove event handling and arrow hit detection, then reproduce the behavior in the first locker room, including the left arrow and nearby trashcan. Done should mean dragging over an inventory arrow works reliably and does not leave the adjacent item glowing, but the desired UX redesign still needs to be decided.

Written by the indexing model from the issue text.