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>

src/components/themes/lyra/LyraTheme.cpp

@@ -274,11 +274,10 @@ void LyraTheme::drawRecentBookCover(GfxRenderer& renderer, Rect rect, const std:
       for (int i = 0; i < std::min(static_cast<int>(recentBooks.size()), LyraMetrics::values.homeRecentBooksCount);
            i++) {
         std::string coverPath = recentBooks[i].coverBmpPath;
-        bool hasCover = true;
         int tileX = LyraMetrics::values.contentSidePadding + tileWidth * i;
-        if (coverPath.empty()) {
-          hasCover = false;
-        } else {
+        renderer.drawRect(tileX + hPaddingInSelection, tileY + hPaddingInSelection,
+                          tileWidth - 2 * hPaddingInSelection, LyraMetrics::values.homeCoverHeight);
+        if (!coverPath.empty()) {
           const std::string coverBmpPath = UITheme::getCoverThumbPath(coverPath, LyraMetrics::values.homeCoverHeight);
 
           // First time: load cover from SD and render
@@ -292,20 +291,12 @@ void LyraTheme::drawRecentBookCover(GfxRenderer& renderer, Rect rect, const std:
               const float tileRatio = static_cast<float>(tileWidth - 2 * hPaddingInSelection) /
                                       static_cast<float>(LyraMetrics::values.homeCoverHeight);
               float cropX = 1.0f - (tileRatio / ratio);
-
               renderer.drawBitmap(bitmap, tileX + hPaddingInSelection, tileY + hPaddingInSelection,
                                   tileWidth - 2 * hPaddingInSelection, LyraMetrics::values.homeCoverHeight, cropX);
-            } else {
-              hasCover = false;
             }
             file.close();
           }
         }
-
-        if (!hasCover) {
-          renderer.drawRect(tileX + hPaddingInSelection, tileY + hPaddingInSelection,
-                            tileWidth - 2 * hPaddingInSelection, LyraMetrics::values.homeCoverHeight);
-        }
       }
 
       coverBufferStored = storeCoverBuffer();