Phanpy strips zero witdh spaces

### Site

phanpy.squirrel.rocks

### Version

2025.04.28.5849b4d

### Instance

boosts.squirrel.pictures

### Browser

Firefox 138.0.4 on openSUSE Tumbleweed / Xfce 4.20

### Bug description

When trying to compose messages containing unicode characters Phanpy does not allow zero width spaces.

When trying to paste text containing zero width spaces, the spaces are stripped.

### To reproduce

1. Open the compose window
2. Try to write a fraction like 5​1⁄4
(5 + ZERO WIDTH SPACE [U+200B] + 1 + FRACTION SLASH [U+2044] + 4)
3. Observe Phanpy quietly dropping the zero with space, making the 5 part of the numerator instead of a separate number

https://github.com/user-attachments/assets/c7256448-587d-41ab-aee6-d392fe58ac3c

### Expected behavior

The input is not modified.

### Other

_No response_