refactor: Removed redundant FsFile close() calls (#1434)

## Summary

**What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)

`DESTRUCTOR_CLOSES_FILE=1` is set in platformio.ini, which makes SdFat's
FsBaseFile destructor call close() automatically when a file goes out of
scope.

Three categories of file close calls remain untouched:
1. Close before Storage.remove() on the same path: ScreenshotUtil.cpp
closes the file before deleting it on write error. The remove might fail
if the file is still open.
2. Close before reopening the same variable: Epub.cpp writes a temp
NCX/nav file, closes it, then reopens it for reading. The
RecentBooksStore.cpp close before saveToFile() is the same pattern, it
rewrites the same file.
3. Close on member variables: BookMetadataCache.cpp (bookFile,
spineFile, tocFile), Section.cpp (file), XtcParser.cpp (m_file),
ZipFile.cpp (file). These persist beyond any single function scope, so
the destructor timing doesn't match the intended close point.

---

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src/activities/reader/EpubReaderActivity.cpp

@@ -69,7 +69,6 @@ void EpubReaderActivity::onEnter() {
     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.
   // This will trigger if the book is re-opened at Chapter 0.
@@ -696,7 +695,6 @@ void EpubReaderActivity::saveProgress(int spineIndex, int currentPage, int pageC
     data[4] = pageCount & 0xFF;
     data[5] = (pageCount >> 8) & 0xFF;
     f.write(data, 6);
-    f.close();
     LOG_DBG("ERS", "Progress saved: Chapter %d, Page %d", spineIndex, currentPage);
   } else {
     LOG_ERR("ERS", "Could not save progress!");