No Pinch to Zoom

## Description

Pinch to Zoom gestures don't work. My touchpad is capable of this, and other apps supposedly support it. I've tried it with firefox (using the MOZ_XINPUT_2 variable) and DevHelp and they will zoom in when pinching on the touchpad.

Note: the system is X11, but uses libinput for pinch gestures

### Steps to reproduce the bug

1. Enable CTRL to Zoom in Drawing Preferences.
2. Try to zoom on the touchpad.
3. Only CTRL + moving fingers like a scroll wheel works.

## System

- Device (desktop computer/tablet/phone/...): laptop computer
- OS: Linux Mint 22 Cinnamon
- Desktop environment: Cinnamon
- Package format: Native builtin package
- App version: 1.0.2

Start by reproducing the gesture with CTRL to Zoom enabled in Drawing Preferences on the reported Linux Mint X11/libinput setup. Trace the app's touchpad and zoom input handling, then verify that a pinch zooms the canvas while the existing CTRL-plus-finger movement behavior still works.

Written by the indexing model from the issue text.