Tiles are 4 pixels off-center

### Steps to reproduce

1. Join a call
2. Use grid layout
3. Stare at the interface reeeally closely

### Outcome

#### What did you expect?

Tile should be centered in the middle of the screen

#### What happened instead?

The tile is closer to the right edge of the screen than the left. This is a regression due to the addition of `margin: var(--hover-space-margin);` in f509c06cc.

### Operating system

NixOS 25.05

### Browser information

Firefox 141.0.3

### URL for webapp

call.element.dev

### Will you send logs?

No

Inspect commit f509c06cc and the grid-layout styling it changed, focusing on the added `margin: var(--hover-space-margin);`. Reproduce the layout at call.element.dev and confirm the tile has equal space on both sides; done means the grid tile is centered without regressing the intended hover spacing.

Written by the indexing model from the issue text.