feat(home): Add cover image thumbnail to Continue Reading card

Display book cover image as background in the Continue Reading card on
the home screen, improving visual identification of the current book.

Key changes:
- Add thumbnail generation (thumb.bmp) for EPUB and XTC/XTCH files
  - Uses same dithering/scaling algorithms as sleep screen covers
  - Target size: 240x400 (half screen) for optimal Continue Reading card fit
- Add JpegToBmpConverter::jpegFileToBmpStreamWithSize() for custom target sizes
- Add GfxRenderer::copyStoredBwBuffer() and freeStoredBwBuffer() for
  framebuffer caching to maintain fast navigation performance
- Add UTF-8 safe string truncation for Korean/CJK text in title/author display
- Draw white boxes behind title/author text for readability over cover image
- Increase HomeActivityTask stack size to 4096 for cover image rendering
- Add bounds checking in XTC thumbnail generation to prevent buffer overflow

lib/Epub/Epub.cpp

@@ -328,6 +328,69 @@ bool Epub::generateCoverBmp() 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)
+    constexpr int THUMB_TARGET_WIDTH = 240;
+    constexpr int THUMB_TARGET_HEIGHT = 400;
+    const bool success =
+        JpegToBmpConverter::jpegFileToBmpStreamWithSize(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());

lib/Epub/Epub.h

@@ -43,6 +43,8 @@ class Epub {
   const std::string& getAuthor() const;
   std::string getCoverBmpPath() const;
   bool generateCoverBmp() const;
+  std::string getThumbBmpPath() const;
+  bool generateThumbBmp() const;
   uint8_t* readItemContentsToBytes(const std::string& itemHref, size_t* size = nullptr,
                                    bool trailingNullByte = false) const;
   bool readItemContentsToStream(const std::string& itemHref, Print& out, size_t chunkSize) const;