Add cover image display in *Continue Reading* card with framebuffer caching (#200)

## Summary

* **What is the goal of this PR?** (e.g., Fixes a bug in the user
authentication module,

Display the book cover image in the **"Continue Reading"** card on the
home screen, with fast navigation using framebuffer caching.

* **What changes are included?**

- Display book cover image in the "Continue Reading" card on home screen
- Load cover from cached BMP (same as sleep screen cover)
- Add framebuffer store/restore functions (`copyStoredBwBuffer`,
`freeStoredBwBuffer`) for fast navigation after initial render
- Fix `drawBitmap` scaling bug: apply scale to offset only, not to base
coordinates
- Add white text boxes behind title/author/continue reading label for
readability on cover
- Support both EPUB and XTC file cover images
- Increase HomeActivity task stack size from 2048 to 4096 for cover
image rendering

## Additional Context

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

- Performance: First render loads cover from SD card (~800ms),
subsequent navigation uses cached framebuffer (~instant)
- Memory: Framebuffer cache uses ~48KB (6 chunks × 8KB) while on home
screen, freed on exit
- Fallback: If cover image is not available, falls back to standard
text-only display
- The `drawBitmap` fix corrects a bug where screenY = (y + offset) scale
was incorrectly scaling the base coordinates. Now correctly uses screenY
= y + (offset scale)

lib/Epub/Epub.cpp

@@ -409,6 +409,70 @@ bool Epub::generateCoverBmp(bool cropped) const {
   return false;
 }
 
+std::string Epub::getThumbBmpPath() const { return cachePath + "/thumb.bmp"; }
+
+bool Epub::generateThumbBmp() const {
+  // Already generated, return true
+  if (SdMan.exists(getThumbBmpPath().c_str())) {
+    return true;
+  }
+
+  if (!bookMetadataCache || !bookMetadataCache->isLoaded()) {
+    Serial.printf("[%lu] [EBP] Cannot generate thumb BMP, cache not loaded\n", millis());
+    return false;
+  }
+
+  const auto coverImageHref = bookMetadataCache->coreMetadata.coverItemHref;
+  if (coverImageHref.empty()) {
+    Serial.printf("[%lu] [EBP] No known cover image for thumbnail\n", millis());
+    return false;
+  }
+
+  if (coverImageHref.substr(coverImageHref.length() - 4) == ".jpg" ||
+      coverImageHref.substr(coverImageHref.length() - 5) == ".jpeg") {
+    Serial.printf("[%lu] [EBP] Generating thumb BMP from JPG cover image\n", millis());
+    const auto coverJpgTempPath = getCachePath() + "/.cover.jpg";
+
+    FsFile coverJpg;
+    if (!SdMan.openFileForWrite("EBP", coverJpgTempPath, coverJpg)) {
+      return false;
+    }
+    readItemContentsToStream(coverImageHref, coverJpg, 1024);
+    coverJpg.close();
+
+    if (!SdMan.openFileForRead("EBP", coverJpgTempPath, coverJpg)) {
+      return false;
+    }
+
+    FsFile thumbBmp;
+    if (!SdMan.openFileForWrite("EBP", getThumbBmpPath(), thumbBmp)) {
+      coverJpg.close();
+      return false;
+    }
+    // Use smaller target size for Continue Reading card (half of screen: 240x400)
+    // Generate 1-bit BMP for fast home screen rendering (no gray passes needed)
+    constexpr int THUMB_TARGET_WIDTH = 240;
+    constexpr int THUMB_TARGET_HEIGHT = 400;
+    const bool success = JpegToBmpConverter::jpegFileTo1BitBmpStreamWithSize(coverJpg, thumbBmp, THUMB_TARGET_WIDTH,
+                                                                             THUMB_TARGET_HEIGHT);
+    coverJpg.close();
+    thumbBmp.close();
+    SdMan.remove(coverJpgTempPath.c_str());
+
+    if (!success) {
+      Serial.printf("[%lu] [EBP] Failed to generate thumb BMP from JPG cover image\n", millis());
+      SdMan.remove(getThumbBmpPath().c_str());
+    }
+    Serial.printf("[%lu] [EBP] Generated thumb BMP from JPG cover image, success: %s\n", millis(),
+                  success ? "yes" : "no");
+    return success;
+  } else {
+    Serial.printf("[%lu] [EBP] Cover image is not a JPG, skipping thumbnail\n", millis());
+  }
+
+  return false;
+}
+
 uint8_t* Epub::readItemContentsToBytes(const std::string& itemHref, size_t* size, const bool trailingNullByte) const {
   if (itemHref.empty()) {
     Serial.printf("[%lu] [EBP] Failed to read item, empty href\n", millis());