fix: Port upstream cover extraction fallback and outline improvements

Port PR #838 (epub cover fallback logic) and PR #907 (cover outlines):

- Add fallback cover filename probing when EPUB metadata lacks cover info
- Case-insensitive extension checking for cover images
- Detect and re-generate corrupt/empty thumbnail BMPs
- Always draw outline rect on cover tiles for legibility (PR #907)
- Upgrade Storage.exists() checks to Epub::isValidThumbnailBmp()
- Fallback chain: Real Cover → PlaceholderCoverGenerator → X-pattern marker
- Add epub.load retry logic (cache-only first, then full build)
- Adapt upstream Serial.printf calls to LOG_DBG/LOG_ERR macros

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/Epub/Epub.h

@@ -52,10 +52,23 @@ class Epub {
   const std::string& getAuthor() const;
   const std::string& getLanguage() const;
   std::string getCoverBmpPath(bool cropped = false) const;
+  // Generate a 1-bit BMP cover image from the EPUB cover image.
+  // Returns true on success. On conversion failure, callers may use
+  // `generateInvalidFormatCoverBmp` to create a valid marker BMP.
   bool generateCoverBmp(bool cropped = false) const;
+  // Create a valid 1-bit BMP that visually indicates an invalid/unsupported
+  // cover format (an X pattern). This prevents repeated generation attempts
+  // by providing a valid BMP file that `isValidThumbnailBmp` accepts.
+  bool generateInvalidFormatCoverBmp(bool cropped = false) const;
   std::string getThumbBmpPath() const;
   std::string getThumbBmpPath(int height) const;
+  // Generate a thumbnail BMP at the requested `height`. Returns true on
+  // successful conversion. If conversion fails, `generateInvalidFormatThumbBmp`
+  // can be used to write a valid marker image that prevents retries.
   bool generateThumbBmp(int height) const;
+  // Create a valid 1-bit thumbnail BMP with an X marker indicating an
+  // invalid/unsupported cover image instead of leaving an empty marker file.
+  bool generateInvalidFormatThumbBmp(int height) 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;
@@ -72,4 +85,9 @@ class Epub {
   size_t getBookSize() const;
   float calculateProgress(int currentSpineIndex, float currentSpineRead) const;
   CssParser* getCssParser() const { return cssParser.get(); }
+
+  static bool isValidThumbnailBmp(const std::string& bmpPath);
+
+ private:
+  std::vector<std::string> getCoverCandidates() const;
 };