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/CrossPointSettings.h

@@ -90,6 +90,8 @@ class CrossPointSettings {
   char opdsServerUrl[128] = "";
   // Hide battery percentage
   uint8_t hideBatteryPercentage = HIDE_NEVER;
+  // Long-press chapter skip on side buttons
+  uint8_t longPressChapterSkip = 1;
 
   ~CrossPointSettings() = default;