Add TXT file reader support

- Add Txt library for loading and parsing plain text files
- Create TxtReaderActivity with streaming page rendering
  - Uses 8KB chunks to handle large files without memory issues
  - Page index caching for fast re-open after sleep
  - Progress bar during initial indexing
  - Word wrapping with UTF-8 support
- Support cover images for TXT files
  - Priority: same filename as TXT (e.g., book.jpg for book.txt)
  - Fallback: cover.bmp/jpg/jpeg in same folder
  - Converts JPG to BMP using existing converter
- Update SleepActivity to show TXT cover images in Cover mode
- Add .txt extension to file browser filter

src/activities/reader/FileSelectionActivity.cpp

@@ -53,7 +53,7 @@ void FileSelectionActivity::loadFiles() {
       auto filename = std::string(name);
       std::string ext4 = filename.length() >= 4 ? filename.substr(filename.length() - 4) : "";
       std::string ext5 = filename.length() >= 5 ? filename.substr(filename.length() - 5) : "";
-      if (ext5 == ".epub" || ext5 == ".xtch" || ext4 == ".xtc") {
+      if (ext5 == ".epub" || ext5 == ".xtch" || ext4 == ".xtc" || ext4 == ".txt") {
         files.emplace_back(filename);
       }
     }