feat: Add placeholder cover generator for books without covers

Generate styled placeholder covers (title, author, book icon) when a
book has no embedded cover image, instead of showing a blank rectangle.

- Add PlaceholderCoverGenerator lib with 1-bit BMP rendering, scaled
  fonts, word-wrap, and a book icon bitmap
- Integrate as fallback in Epub/Xtc/Txt reader activities and
  SleepActivity after format-specific cover generation fails
- Add fallback in HomeActivity::loadRecentCovers() so the home screen
  also shows placeholder thumbnails when cache is cleared
- Add Txt::getThumbBmpPath() for TXT thumbnail support
- Add helper scripts for icon and layout preview generation

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

src/activities/reader/EpubReaderActivity.cpp

@@ -6,6 +6,8 @@
 #include <HalStorage.h>
 #include <Logging.h>
 
+#include <PlaceholderCoverGenerator.h>
+
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
 #include "EpubReaderBookmarkSelectionActivity.h"
@@ -126,15 +128,31 @@ void EpubReaderActivity::onEnter() {
 
       if (!Storage.exists(epub->getCoverBmpPath(false).c_str())) {
         epub->generateCoverBmp(false);
+        // Fallback: generate placeholder if real cover extraction failed
+        if (!Storage.exists(epub->getCoverBmpPath(false).c_str())) {
+          PlaceholderCoverGenerator::generate(epub->getCoverBmpPath(false), epub->getTitle(), epub->getAuthor(), 480,
+                                             800);
+        }
         updateProgress();
       }
       if (!Storage.exists(epub->getCoverBmpPath(true).c_str())) {
         epub->generateCoverBmp(true);
+        if (!Storage.exists(epub->getCoverBmpPath(true).c_str())) {
+          PlaceholderCoverGenerator::generate(epub->getCoverBmpPath(true), epub->getTitle(), epub->getAuthor(), 480,
+                                             800);
+        }
         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]);
+          // Fallback: generate placeholder thumbnail
+          if (!Storage.exists(epub->getThumbBmpPath(PRERENDER_THUMB_HEIGHTS[i]).c_str())) {
+            const int thumbHeight = PRERENDER_THUMB_HEIGHTS[i];
+            const int thumbWidth = static_cast<int>(thumbHeight * 0.6);
+            PlaceholderCoverGenerator::generate(epub->getThumbBmpPath(thumbHeight), epub->getTitle(),
+                                               epub->getAuthor(), thumbWidth, thumbHeight);
+          }
           updateProgress();
         }
       }

src/activities/reader/TxtReaderActivity.cpp

@@ -5,6 +5,8 @@
 #include <Serialization.h>
 #include <Utf8.h>
 
+#include <PlaceholderCoverGenerator.h>
+
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
 #include "MappedInputManager.h"
@@ -57,13 +59,43 @@ 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);
+  // 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(txt->getCoverBmpPath().c_str())) totalSteps++;
+    for (int i = 0; i < PRERENDER_THUMB_HEIGHTS_COUNT; i++) {
+      if (!Storage.exists(txt->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(txt->getCoverBmpPath().c_str())) {
+        const bool coverGenerated = txt->generateCoverBmp();
+        // Fallback: generate placeholder if no cover image was found
+        if (!coverGenerated) {
+          PlaceholderCoverGenerator::generate(txt->getCoverBmpPath(), txt->getTitle(), "", 480, 800);
+        }
+        updateProgress();
+      }
+      for (int i = 0; i < PRERENDER_THUMB_HEIGHTS_COUNT; i++) {
+        if (!Storage.exists(txt->getThumbBmpPath(PRERENDER_THUMB_HEIGHTS[i]).c_str())) {
+          // TXT has no native thumbnail generation, always use placeholder
+          const int thumbHeight = PRERENDER_THUMB_HEIGHTS[i];
+          const int thumbWidth = static_cast<int>(thumbHeight * 0.6);
+          PlaceholderCoverGenerator::generate(txt->getThumbBmpPath(thumbHeight), txt->getTitle(), "", thumbWidth,
+                                             thumbHeight);
+          updateProgress();
+        }
+      }
+    }
   }
 
   // Save current txt as last opened file and add to recent books
@@ -71,7 +103,7 @@ void TxtReaderActivity::onEnter() {
   auto fileName = filePath.substr(filePath.rfind('/') + 1);
   APP_STATE.openEpubPath = filePath;
   APP_STATE.saveToFile();
-  RECENT_BOOKS.addBook(filePath, fileName, "", "");
+  RECENT_BOOKS.addBook(filePath, fileName, "", txt->getThumbBmpPath());
 
   // Trigger first update
   updateRequired = true;

src/activities/reader/XtcReaderActivity.cpp

@@ -11,6 +11,8 @@
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 
+#include <PlaceholderCoverGenerator.h>
+
 #include "CrossPointSettings.h"
 #include "CrossPointState.h"
 #include "MappedInputManager.h"
@@ -63,11 +65,22 @@ void XtcReaderActivity::onEnter() {
 
       if (!Storage.exists(xtc->getCoverBmpPath().c_str())) {
         xtc->generateCoverBmp();
+        // Fallback: generate placeholder if first-page cover extraction failed
+        if (!Storage.exists(xtc->getCoverBmpPath().c_str())) {
+          PlaceholderCoverGenerator::generate(xtc->getCoverBmpPath(), xtc->getTitle(), xtc->getAuthor(), 480, 800);
+        }
         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]);
+          // Fallback: generate placeholder thumbnail
+          if (!Storage.exists(xtc->getThumbBmpPath(PRERENDER_THUMB_HEIGHTS[i]).c_str())) {
+            const int thumbHeight = PRERENDER_THUMB_HEIGHTS[i];
+            const int thumbWidth = static_cast<int>(thumbHeight * 0.6);
+            PlaceholderCoverGenerator::generate(xtc->getThumbBmpPath(thumbHeight), xtc->getTitle(), xtc->getAuthor(),
+                                               thumbWidth, thumbHeight);
+          }
           updateProgress();
         }
       }