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>

lib/PlaceholderCover/PlaceholderCoverGenerator.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <string>
+
+/// Generates simple 1-bit BMP placeholder covers with title/author text
+/// for books that have no embedded cover image.
+class PlaceholderCoverGenerator {
+ public:
+  /// Generate a placeholder cover BMP with title and author text.
+  /// The BMP is written to outputPath as a 1-bit black-and-white image.
+  /// Returns true if the file was written successfully.
+  static bool generate(const std::string& outputPath, const std::string& title, const std::string& author, int width,
+                       int height);
+};