prerender book covers and thumbnails when opening a book for the first time

Moves cover/thumbnail generation from lazy (Home screen, Sleep screen) into
each reader activity's onEnter(). On first open, generates all needed BMPs
(cover, cover_crop, thumbnails for all theme heights) with a "Preparing
book..." progress popup. Subsequent opens skip instantly when files exist.

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

src/activities/reader/TxtReaderActivity.cpp

@@ -57,6 +57,15 @@ void TxtReaderActivity::onEnter() {
 
   txt->setupCacheDir();
 
+  // Prerender cover on first open so the Sleep screen is instant.
+  // generateCoverBmp() is a no-op if the file already exists, so this only does work once.
+  // TXT has no thumbnail support, so only the sleep screen cover is generated.
+  if (!Storage.exists(txt->getCoverBmpPath().c_str())) {
+    Rect popupRect = GUI.drawPopup(renderer, "Preparing book...");
+    txt->generateCoverBmp();
+    GUI.fillPopupProgress(renderer, popupRect, 100);
+  }
+
   // Save current txt as last opened file and add to recent books
   auto filePath = txt->getPath();
   auto fileName = filePath.substr(filePath.rfind('/') + 1);