refactor: Simplify XtcReaderActivity with detectPageTurn (#1837)

## Summary

Simplify duplicated code in `XtcReaderActivity` to use
`ReaderUtils::detectPageTurn`. This implementation is now shared with
`EpubReaderActivity` and `TxtReaderActivity`. Also deduplicated the
chapter skip time constant.

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src/activities/reader/EpubReaderActivity.cpp

@@ -30,7 +30,6 @@
 
 namespace {
 // pagesPerRefresh now comes from SETTINGS.getRefreshFrequency()
-constexpr unsigned long skipChapterMs = 700;
 // pages per minute, first item is 1 to prevent division by zero if accessed
 constexpr int PAGE_TURN_RATES[] = {1, 1, 3, 6, 12};
 
@@ -185,7 +184,7 @@ void EpubReaderActivity::loop() {
     return;
   }
 
-  auto [prevTriggered, nextTriggered, fromTilt] = ReaderUtils::detectPageTurn(mappedInput);
+  const auto [prevTriggered, nextTriggered, fromTilt] = ReaderUtils::detectPageTurn(mappedInput);
   if (!prevTriggered && !nextTriggered) {
     return;
   }
@@ -203,7 +202,7 @@ void EpubReaderActivity::loop() {
     return;
   }
 
-  const bool longPress = !fromTilt && mappedInput.getHeldTime() > skipChapterMs;
+  const bool longPress = !fromTilt && mappedInput.getHeldTime() > ReaderUtils::SKIP_HOLD_MS;
 
   // Don't skip chapter after screenshot
   if (gpio.wasReleased(HalGPIO::BTN_POWER) && gpio.wasReleased(HalGPIO::BTN_DOWN)) {