Inconsistent mini-widget placement in top and bottom bars

[ES-Alexander]

Bug description

Mini (and input) widget placement fails at the transitions between the containers in each bar:

https://github.com/user-attachments/assets/b1bedfa0-1c97-4716-a994-492ad91e6aba

Steps to reproduce

1. Open Edit Mode
2. Drag a mini-widget or input widget to a container boundary in the top or bottom bar
3. Move it around - it does not "enter" the container until right up against an existing widget

Primary pain point(s)

It's unintuitive and can be frustrating that parts of the bar just don't allow widgets, for no discernible reason.

Ideally when dragging a widget to enter the bar, the containers should be considered to expand equally into the space between each other (which is currently created using <div class="grow"> elements), so if a widget is in the left half of the space it gets considered as part of the container to its left, and vice versa for the right side.

Additional context

Discovered in this PR review

Prerequisites

• [x] I have checked to make sure that a similar request has not already been filed or fixed.