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/util/ScreenshotUtil.cpp

@@ -70,6 +70,7 @@ bool ScreenshotUtil::saveFramebufferAsBmp(const char* filename, const uint8_t* f
   }
 
   if (write_error) {
+    // Explicitly close() file before calling Storage.remove()
     file.close();
     Storage.remove(filename);
     return false;
@@ -80,6 +81,7 @@ bool ScreenshotUtil::saveFramebufferAsBmp(const char* filename, const uint8_t* f
   constexpr size_t kMaxRowSize = 68;
   if (rowSizePadded > kMaxRowSize) {
     LOG_ERR("SCR", "Row size %u exceeds buffer capacity", rowSizePadded);
+    // Explicitly close() file before calling Storage.remove()
     file.close();
     Storage.remove(filename);
     return false;
@@ -106,6 +108,7 @@ bool ScreenshotUtil::saveFramebufferAsBmp(const char* filename, const uint8_t* f
     memset(rowBuffer, 0, rowSizePadded);  // Clear the buffer for the next row
   }
 
+  // Explicitly close() file before calling Storage.remove()
   file.close();
 
   if (write_error) {