fix: Scrolling page items calculation (#716)

## Summary

Fix for the page skip issue detected
https://github.com/crosspoint-reader/crosspoint-reader/pull/700#issuecomment-3856374323
by user @whyte-j

Skipping down on the last page now skips to the last item, and up on the
first page to the first item, rather than wrapping around the list in a
weird way.

## Additional Context

The calculation was outdated after several changes were added afterwards

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**NO**_

src/components/UITheme.cpp

@@ -40,10 +40,10 @@ int UITheme::getNumberOfItemsPerPage(const GfxRenderer& renderer, bool hasHeader
   const ThemeMetrics& metrics = UITheme::getInstance().getMetrics();
   int reservedHeight = metrics.topPadding;
   if (hasHeader) {
-    reservedHeight += metrics.headerHeight;
+    reservedHeight += metrics.headerHeight + metrics.verticalSpacing;
   }
   if (hasTabBar) {
-    reservedHeight += metrics.tabBarHeight + metrics.verticalSpacing;
+    reservedHeight += metrics.tabBarHeight;
   }
   if (hasButtonHints) {
     reservedHeight += metrics.verticalSpacing + metrics.buttonHintsHeight;

src/components/themes/BaseTheme.cpp

@@ -174,7 +174,7 @@ void BaseTheme::drawList(const GfxRenderer& renderer, Rect rect, int itemCount,
 
     const int centerX = rect.x + rect.width - indicatorWidth / 2 - margin;
     const int indicatorTop = rect.y;  // Offset to avoid overlapping side button hints
-    const int indicatorBottom = rect.y + rect.height - 30;
+    const int indicatorBottom = rect.y + rect.height - arrowSize;
 
     // Draw up arrow at top (^) - narrow point at top, wide base at bottom
     for (int i = 0; i < arrowSize; ++i) {