Large space hierarchies get badly horizontally truncated by not being able to resize the SpacePanel

### Steps to reproduce

1. Create a large space hierarchy, 4-5 levels deep
2. Join the spaces & subspaces
3. Observe that the names of the subspaces get badly truncated by the SpacePanel only being 250px wide
4. Have a claustrophobic panic attack
5. Manually override `.mx_SpacePanel .mx_SpaceTreeLevel { max-width }` to be 350px wide
6. Wish that you could resize the SpacePanel by dragging, like you can the LeftPanel & RightPanel.

### Outcome

#### What did you expect?

Resizable SpacePanel

#### What happened instead?

claustrophobia

### Operating system

macOS

### Browser information

nightly

### URL for webapp

_No response_

### Application version

nightly

### Homeserver

matrix.org

### Will you send logs?

No