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/activities/boot_sleep/SleepActivity.cpp

@@ -642,11 +642,14 @@ void SleepActivity::renderCoverSleepScreen() const {
 
     if (!lastEpub.generateCoverBmp(cropped)) {
       LOG_DBG("SLP", "EPUB cover generation failed, trying placeholder");
-      PlaceholderCoverGenerator::generate(lastEpub.getCoverBmpPath(cropped), lastEpub.getTitle(),
-                                         lastEpub.getAuthor(), 480, 800);
+      if (!PlaceholderCoverGenerator::generate(lastEpub.getCoverBmpPath(cropped), lastEpub.getTitle(),
+                                               lastEpub.getAuthor(), 480, 800)) {
+        LOG_DBG("SLP", "Placeholder generation failed, creating X-pattern marker");
+        lastEpub.generateInvalidFormatCoverBmp(cropped);
+      }
     }
 
-    if (!Storage.exists(lastEpub.getCoverBmpPath(cropped).c_str())) {
+    if (!Epub::isValidThumbnailBmp(lastEpub.getCoverBmpPath(cropped))) {
       LOG_ERR("SLP", "Failed to generate cover bmp");
       return (this->*renderNoCoverSleepScreen)();
     }