Add retry logic and progress bar for chapter indexing (#128)

## Summary

* **What is the goal of this PR?**

Improve reliability and user experience during chapter indexing by
adding retry logic for SD card operations and a visual progress bar.

* **What changes are included?**

- **Retry logic**: Add 3 retry attempts with 50ms delay for ZIP to SD
card streaming to handle timing issues after display refresh
- **Progress bar**: Display a visual progress bar (0-100%) during
chapter indexing based on file read progress, updating every 10% to
balance responsiveness with e-ink display limitations

## Additional Context

* **Problem observed**: When navigating quickly through books with many
chapters (before chapter titles finish rendering), the "Indexing..."
screen would appear frozen. Checking the serial log revealed the
operation had silently failed, but the UI showed no indication of this.
Users would likely assume the device had crashed. Pressing the next
button again would resume operation, but this behavior was confusing and
unexpected.

* **Solution**:
- Retry logic handles transient SD card timing failures automatically,
so users don't need to manually retry
- Progress bar provides visual feedback so users know indexing is
actively working (not frozen)

* **Why timing issues occur**: After display refresh operations, there
can be timing conflicts when immediately starting SD card write
operations. This is more likely to happen when rapidly navigating
through chapters.

* **Progress bar design**: Updates every 10% to avoid excessive e-ink
refreshes while still providing meaningful feedback during long indexing
operations (especially for large chapters with CJK characters).

* **Performance**: Minimal overhead - progress calculation is simple
byte counting, and display updates use `FAST_REFRESH` mode.

lib/Epub/Epub/Section.cpp

@@ -115,26 +115,56 @@ bool Section::clearCache() const {
 
 bool Section::persistPageDataToSD(const int fontId, const float lineCompression, const int marginTop,
                                   const int marginRight, const int marginBottom, const int marginLeft,
-                                  const bool extraParagraphSpacing) {
+                                  const bool extraParagraphSpacing, const std::function<void()>& progressSetupFn,
+                                  const std::function<void(int)>& progressFn) {
+  constexpr size_t MIN_SIZE_FOR_PROGRESS = 50 * 1024;  // 50KB
   const auto localPath = epub->getSpineItem(spineIndex).href;
   const auto tmpHtmlPath = epub->getCachePath() + "/.tmp_" + std::to_string(spineIndex) + ".html";
-  File tmpHtml;
-  if (!FsHelpers::openFileForWrite("SCT", tmpHtmlPath, tmpHtml)) {
-    return false;
+
+  // Retry logic for SD card timing issues
+  bool success = false;
+  size_t fileSize = 0;
+  for (int attempt = 0; attempt < 3 && !success; attempt++) {
+    if (attempt > 0) {
+      Serial.printf("[%lu] [SCT] Retrying stream (attempt %d)...\n", millis(), attempt + 1);
+      delay(50);  // Brief delay before retry
+    }
+
+    // Remove any incomplete file from previous attempt before retrying
+    if (SD.exists(tmpHtmlPath.c_str())) {
+      SD.remove(tmpHtmlPath.c_str());
+    }
+
+    File tmpHtml;
+    if (!FsHelpers::openFileForWrite("SCT", tmpHtmlPath, tmpHtml)) {
+      continue;
+    }
+    success = epub->readItemContentsToStream(localPath, tmpHtml, 1024);
+    fileSize = tmpHtml.size();
+    tmpHtml.close();
+
+    // If streaming failed, remove the incomplete file immediately
+    if (!success && SD.exists(tmpHtmlPath.c_str())) {
+      SD.remove(tmpHtmlPath.c_str());
+      Serial.printf("[%lu] [SCT] Removed incomplete temp file after failed attempt\n", millis());
+    }
   }
-  bool success = epub->readItemContentsToStream(localPath, tmpHtml, 1024);
-  tmpHtml.close();
 
   if (!success) {
-    Serial.printf("[%lu] [SCT] Failed to stream item contents to temp file\n", millis());
+    Serial.printf("[%lu] [SCT] Failed to stream item contents to temp file after retries\n", millis());
     return false;
   }
 
-  Serial.printf("[%lu] [SCT] Streamed temp HTML to %s\n", millis(), tmpHtmlPath.c_str());
+  Serial.printf("[%lu] [SCT] Streamed temp HTML to %s (%d bytes)\n", millis(), tmpHtmlPath.c_str(), fileSize);
 
-  ChapterHtmlSlimParser visitor(tmpHtmlPath, renderer, fontId, lineCompression, marginTop, marginRight, marginBottom,
-                                marginLeft, extraParagraphSpacing,
-                                [this](std::unique_ptr<Page> page) { this->onPageComplete(std::move(page)); });
+  // Only show progress bar for larger chapters where rendering overhead is worth it
+  if (progressSetupFn && fileSize >= MIN_SIZE_FOR_PROGRESS) {
+    progressSetupFn();
+  }
+
+  ChapterHtmlSlimParser visitor(
+      tmpHtmlPath, renderer, fontId, lineCompression, marginTop, marginRight, marginBottom, marginLeft,
+      extraParagraphSpacing, [this](std::unique_ptr<Page> page) { this->onPageComplete(std::move(page)); }, progressFn);
   success = visitor.parseAndBuildPages();
 
   SD.remove(tmpHtmlPath.c_str());