Left arrow for boost carousel not working

**Describe the bug**
A clear and concise description of what the bug is.
- Which site: phanpy.social
- Which instance: shelter.moe

**To Reproduce**

1) Have a boost carousel with multiple boosts
2) Click on the right arrow, it goes to the next boost
3) Click on the left arrow, it stays on the current boost

**Expected behavior**
Left arrow should move to the previous boost correctly

**Video**

https://github.com/cheeaun/phanpy/assets/3592205/58c7bbb2-96a1-4481-8c56-93dd3f948413

**Desktop (please complete the following information):**
- OS: W11
- Browser: Firefox
- Version: 119.0