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).

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### AI Usage

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

lib/I18n/translations/english.yaml

@@ -101,7 +101,7 @@ STR_FONT_FAMILY: "Reader Font Family"
 STR_EXT_READER_FONT: "External Reader Font"
 STR_EXT_CHINESE_FONT: "Reader Font"
 STR_EXT_UI_FONT: "UI Font"
-STR_FONT_SIZE: "UI Font Size"
+STR_FONT_SIZE: "Reader Font Size"
 STR_LINE_SPACING: "Reader Line Spacing"
 STR_ASCII_LETTER_SPACING: "ASCII Letter Spacing"
 STR_ASCII_DIGIT_SPACING: "ASCII Digit Spacing"