fix: goes to relative position when reader settings are changed (#486)

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)
* Aims to fix Issue #220 

* **What changes are included?**
- Increased size of `progress.bin` such that total page count of current
section can be stored
- Comparison of total page count is done to determine if reader settings
were changed
- New position/page number is calculated using percentage calculated
from read progress

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

---

### AI Usage

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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/activities/reader/EpubReaderActivity.cpp

@@ -56,12 +56,17 @@ void EpubReaderActivity::onEnter() {
 
   FsFile f;
   if (SdMan.openFileForRead("ERS", epub->getCachePath() + "/progress.bin", f)) {
-    uint8_t data[4];
+    uint8_t data[6];
-    if (f.read(data, 4) == 4) {
+    int dataSize = f.read(data, 6);
+    if (dataSize == 4 || dataSize == 6) {
       currentSpineIndex = data[0] + (data[1] << 8);
       nextPageNumber = data[2] + (data[3] << 8);
+      cachedSpineIndex = currentSpineIndex;
       Serial.printf("[%lu] [ERS] Loaded cache: %d, %d\n", millis(), currentSpineIndex, nextPageNumber);
     }
+    if (dataSize == 6) {
+      cachedChapterTotalPageCount = data[4] + (data[5] << 8);
+    }
     f.close();
   }
   // We may want a better condition to detect if we are opening for the first time.
@@ -341,6 +346,17 @@ void EpubReaderActivity::renderScreen() {
     } else {
       section->currentPage = nextPageNumber;
     }
+
+    // handles changes in reader settings and reset to approximate position based on cached progress
+    if (cachedChapterTotalPageCount > 0) {
+      // only goes to relative position if spine index matches cached value
+      if (currentSpineIndex == cachedSpineIndex && section->pageCount != cachedChapterTotalPageCount) {
+        float progress = static_cast<float>(section->currentPage) / static_cast<float>(cachedChapterTotalPageCount);
+        int newPage = static_cast<int>(progress * section->pageCount);
+        section->currentPage = newPage;
+      }
+      cachedChapterTotalPageCount = 0;  // resets to 0 to prevent reading cached progress again
+    }
   }
 
   renderer.clearScreen();
@@ -376,12 +392,14 @@ void EpubReaderActivity::renderScreen() {
 
   FsFile f;
   if (SdMan.openFileForWrite("ERS", epub->getCachePath() + "/progress.bin", f)) {
-    uint8_t data[4];
+    uint8_t data[6];
     data[0] = currentSpineIndex & 0xFF;
     data[1] = (currentSpineIndex >> 8) & 0xFF;
     data[2] = section->currentPage & 0xFF;
     data[3] = (section->currentPage >> 8) & 0xFF;
-    f.write(data, 4);
+    data[4] = section->pageCount & 0xFF;
+    data[5] = (section->pageCount >> 8) & 0xFF;
+    f.write(data, 6);
     f.close();
   }
 }

src/activities/reader/EpubReaderActivity.h

@@ -15,6 +15,8 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   int currentSpineIndex = 0;
   int nextPageNumber = 0;
   int pagesUntilFullRefresh = 0;
+  int cachedSpineIndex = 0;
+  int cachedChapterTotalPageCount = 0;
   bool updateRequired = false;
   const std::function<void()> onGoBack;
   const std::function<void()> onGoHome;