feat: Go To Position for epubs (#666)

## Summary

* Adds Go To % action in Epub Reader menu with slider style percent
selector

<img width="860" height="1147" alt="image"
src="https://github.com/user-attachments/assets/a38ecc71-429e-40e8-94ac-37fb1509dbd9"
/>

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### 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? _**< PARTIALLY >**_

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/activities/reader/EpubReaderActivity.h

@@ -18,6 +18,11 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   int pagesUntilFullRefresh = 0;
   int cachedSpineIndex = 0;
   int cachedChapterTotalPageCount = 0;
+  // Signals that the next render should reposition within the newly loaded section
+  // based on a cross-book percentage jump.
+  bool pendingPercentJump = false;
+  // Normalized 0.0-1.0 progress within the target spine item, computed from book percentage.
+  float pendingSpineProgress = 0.0f;
   bool updateRequired = false;
   const std::function<void()> onGoBack;
   const std::function<void()> onGoHome;
@@ -29,6 +34,8 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
                       int orientedMarginBottom, int orientedMarginLeft);
   void renderStatusBar(int orientedMarginRight, int orientedMarginBottom, int orientedMarginLeft) const;
   void saveProgress(int spineIndex, int currentPage, int pageCount);
+  // Jump to a percentage of the book (0-100), mapping it to spine and page.
+  void jumpToPercent(int percent);
   void onReaderMenuBack(uint8_t orientation);
   void onReaderMenuConfirm(EpubReaderMenuActivity::MenuAction action);
   void applyOrientation(uint8_t orientation);