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/EpubReaderActivity.cpp

@@ -102,6 +102,42 @@ void EpubReaderActivity::onEnter() {
     }
   }
 
+  // Prerender covers and thumbnails on first open so Home and Sleep screens are instant.
+  // Each generate* call is a no-op if the file already exists, so this only does work once.
+  {
+    int totalSteps = 0;
+    if (!Storage.exists(epub->getCoverBmpPath(false).c_str())) totalSteps++;
+    if (!Storage.exists(epub->getCoverBmpPath(true).c_str())) totalSteps++;
+    for (int i = 0; i < PRERENDER_THUMB_HEIGHTS_COUNT; i++) {
+      if (!Storage.exists(epub->getThumbBmpPath(PRERENDER_THUMB_HEIGHTS[i]).c_str())) totalSteps++;
+    }
+
+    if (totalSteps > 0) {
+      Rect popupRect = GUI.drawPopup(renderer, "Preparing book...");
+      int completedSteps = 0;
+
+      auto updateProgress = [&]() {
+        completedSteps++;
+        GUI.fillPopupProgress(renderer, popupRect, completedSteps * 100 / totalSteps);
+      };
+
+      if (!Storage.exists(epub->getCoverBmpPath(false).c_str())) {
+        epub->generateCoverBmp(false);
+        updateProgress();
+      }
+      if (!Storage.exists(epub->getCoverBmpPath(true).c_str())) {
+        epub->generateCoverBmp(true);
+        updateProgress();
+      }
+      for (int i = 0; i < PRERENDER_THUMB_HEIGHTS_COUNT; i++) {
+        if (!Storage.exists(epub->getThumbBmpPath(PRERENDER_THUMB_HEIGHTS[i]).c_str())) {
+          epub->generateThumbBmp(PRERENDER_THUMB_HEIGHTS[i]);
+          updateProgress();
+        }
+      }
+    }
+  }
+
   // Save current epub as last opened epub and add to recent books
   APP_STATE.openEpubPath = epub->getPath();
   APP_STATE.saveToFile();

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);

src/activities/reader/XtcReaderActivity.cpp

@@ -43,6 +43,37 @@ void XtcReaderActivity::onEnter() {
   // Load saved progress
   loadProgress();
 
+  // Prerender covers and thumbnails on first open so Home and Sleep screens are instant.
+  // Each generate* call is a no-op if the file already exists, so this only does work once.
+  {
+    int totalSteps = 0;
+    if (!Storage.exists(xtc->getCoverBmpPath().c_str())) totalSteps++;
+    for (int i = 0; i < PRERENDER_THUMB_HEIGHTS_COUNT; i++) {
+      if (!Storage.exists(xtc->getThumbBmpPath(PRERENDER_THUMB_HEIGHTS[i]).c_str())) totalSteps++;
+    }
+
+    if (totalSteps > 0) {
+      Rect popupRect = GUI.drawPopup(renderer, "Preparing book...");
+      int completedSteps = 0;
+
+      auto updateProgress = [&]() {
+        completedSteps++;
+        GUI.fillPopupProgress(renderer, popupRect, completedSteps * 100 / totalSteps);
+      };
+
+      if (!Storage.exists(xtc->getCoverBmpPath().c_str())) {
+        xtc->generateCoverBmp();
+        updateProgress();
+      }
+      for (int i = 0; i < PRERENDER_THUMB_HEIGHTS_COUNT; i++) {
+        if (!Storage.exists(xtc->getThumbBmpPath(PRERENDER_THUMB_HEIGHTS[i]).c_str())) {
+          xtc->generateThumbBmp(PRERENDER_THUMB_HEIGHTS[i]);
+          updateProgress();
+        }
+      }
+    }
+  }
+
   // Save current XTC as last opened book and add to recent books
   APP_STATE.openEpubPath = xtc->getPath();
   APP_STATE.saveToFile();

src/components/UITheme.h

@@ -27,5 +27,10 @@ class UITheme {
   const BaseTheme* currentTheme;
 };
 
+// Known theme thumbnail heights to prerender when opening a book for the first time.
+// These correspond to homeCoverHeight values across all themes (Lyra=226, Base=400).
+static constexpr int PRERENDER_THUMB_HEIGHTS[] = {226, 400};
+static constexpr int PRERENDER_THUMB_HEIGHTS_COUNT = 2;
+
 // Helper macro to access current theme
 #define GUI UITheme::getInstance().getTheme()