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/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();
         }
       }