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/boot_sleep/SleepActivity.cpp

@@ -4,6 +4,7 @@
 #include <GfxRenderer.h>
 #include <HalStorage.h>
 #include <Logging.h>
+#include <PlaceholderCoverGenerator.h>
 #include <Serialization.h>
 #include <Txt.h>
 #include <Xtc.h>
@@ -599,6 +600,11 @@ void SleepActivity::renderCoverSleepScreen() const {
     }
 
     if (!lastXtc.generateCoverBmp()) {
+      LOG_DBG("SLP", "XTC cover generation failed, trying placeholder");
+      PlaceholderCoverGenerator::generate(lastXtc.getCoverBmpPath(), lastXtc.getTitle(), lastXtc.getAuthor(), 480, 800);
+    }
+
+    if (!Storage.exists(lastXtc.getCoverBmpPath().c_str())) {
       LOG_ERR("SLP", "Failed to generate XTC cover bmp");
       return (this->*renderNoCoverSleepScreen)();
     }
@@ -614,6 +620,11 @@ void SleepActivity::renderCoverSleepScreen() const {
     }
 
     if (!lastTxt.generateCoverBmp()) {
+      LOG_DBG("SLP", "TXT cover generation failed, trying placeholder");
+      PlaceholderCoverGenerator::generate(lastTxt.getCoverBmpPath(), lastTxt.getTitle(), "", 480, 800);
+    }
+
+    if (!Storage.exists(lastTxt.getCoverBmpPath().c_str())) {
       LOG_ERR("SLP", "No cover image found for TXT file");
       return (this->*renderNoCoverSleepScreen)();
     }
@@ -630,6 +641,12 @@ 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 (!Storage.exists(lastEpub.getCoverBmpPath(cropped).c_str())) {
       LOG_ERR("SLP", "Failed to generate cover bmp");
       return (this->*renderNoCoverSleepScreen)();
     }