fix: Change "UI Font Size" to "Reader Font Size" (#1171)

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)
Update "UI Font Size" to "Reader Font Size", to match the rest of the
"Reader" settings and clarify that the setting doesn't change the UI
font.

* **What changes are included?**
Changes the `english.yaml` string and USER_GUIDE.md entry.

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

---

### AI Usage

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Did you use AI tools to help write this code? _**< YES | PARTIALLY | NO
>**_
NO

USER_GUIDE.md

@@ -156,7 +156,7 @@ The Settings screen allows you to configure the device's behavior. There are a f
   - "Bookerly" (default) - Amazon's reading font
   - "Noto Sans" - Google's sans-serif font
   - "Open Dyslexic" - Font designed for readers with dyslexia
-- **UI Font Size**: Adjust the text size for reading; options are "Small", "Medium" (default), "Large", or "X Large".
+- **Reader Font Size**: Adjust the text size for reading; options are "Small", "Medium" (default), "Large", or "X Large".
 
 - **Reader Line Spacing**: Adjust the spacing between lines; options are "Tight", "Normal" (default), or "Wide".
 - **Reader Screen Margin**: Controls the screen margins in Reading Mode between 5 and 40 pixels in 5-pixel increments.