feat: Add setting to toggle long-press chapter skip (#341)

## Summary

Adds a new "Long-press Chapter Skip" toggle in Settings to control
whether holding the side buttons skips chapters.

I kept accidentally triggering chapter skips while reading, which caused
me to lose my place in the middle of long chapters.

## 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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src/activities/reader/EpubReaderActivity.cpp

@@ -168,7 +168,7 @@ void EpubReaderActivity::loop() {
     return;
   }
 
-  const bool skipChapter = mappedInput.getHeldTime() > skipChapterMs;
+  const bool skipChapter = SETTINGS.longPressChapterSkip && mappedInput.getHeldTime() > skipChapterMs;
 
   if (skipChapter) {
     // We don't want to delete the section mid-render, so grab the semaphore