Inappropriate UX for text size.

**Describe the bug**
The text size configurable currently has an inappropriate user experience.
- Which site: phanpy.social
- Which instance: not relevant

**To Reproduce**
Steps to reproduce the behavior:
1. Click on the hamburger menu.
2. Click on 'Settings'.
3. Observe that 'text size' is a slider with five positions.

**Expected behavior**
Text size should be an actual number input, either a spin-box (number box with up/down arrows) or a regular number box.

**Screenshots**

**Desktop (please complete the following information):**
- OS: macOS Ventura 13.4.1
- Browser: Firefox
- Version: 119.0

**Additional context**
This is an issue because browser zoom is not a suitable replacement for freely choosing a font size that is usable by the user, lacking among other things the ability to ensure click-targets remain a reasonable size. I recognise that I could apply userCSS, but this is not a reasonable approach either as most visually-impaired/blind users are not going to be technically competent enough for that to be an option. I also recognise that other applications think that a slider is an appropriate interface; they are wrong, too.