Compose window uses absolute /compose URL path instead of relative path

### Site

(self hosted)

### Version

2025.06.12.43d0d1e

### Instance

(self hosted)

### Browser

Firefox 139.0.1 aarch64 on MacOs

### Bug description

With phanpy unpacked into `~/public_html`, I access it via `https://MACHINE/~USERNAME/phanpy/`. Opening the compose panel and then clicking the "pop out" icon to transform it into a standalone window produces a new window on `https://MACHINE/compose/`, where it should open on `https://MACHINE/~USERNAME/phanpy/compose/`.

### To reproduce

1. Unpack static distribution of phanpy into some non-root URL path of your host. Log in etc.
2. Open the compose panel
3. Click the pop-out icon
4. New window opens on `https://MACHINE/compose/`

### Expected behavior

New window on `https://MACHINE/~USERNAME/phanpy/compose/`

### Other

_No response_