Broken resize handle in markdown editor

### Describe the bug
In the basic markdown editor, a handle appears in the bottom right corner to resize the typing box. When dragging on this handle, the interface breaks. This affects both the side by side and fullscreen editors.

### Expected behavior
I would expect there to not be a handle in editor mode. In side by side mode, I would expect no handle or the handle to cleanly adjust the space given to the editor versus the preview.

### Screenshots

### Server details
* **server OS / version:** archlinux
* **what copyparty did you grab:** arch
* **how you're running it:** systemd service
* run copyparty with `--version` and grab the last 3 lines (they start with `copyparty`, `CPython`, `sqlite`) and paste them below this line:

```
copyparty v1.20.6 "sftp is fine too" (2026-01-31)
CPython v3.14.2 on Linux64 [GCC 15.2.1 20251112]
sqlite 3.51.2*1 | jinja 3.1.6 | pyftpd (None) | tftp (None) | miko (None)
```

### Client details
* OS version: Arch Linux, KDE Plasma
* Browser version: Firefox 146.0.1