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

lib/Txt/Txt.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <SDCardManager.h>
+
+#include <memory>
+#include <string>
+
+class Txt {
+  std::string filepath;
+  std::string cacheBasePath;
+  std::string cachePath;
+  bool loaded = false;
+  size_t fileSize = 0;
+
+ public:
+  explicit Txt(std::string path, std::string cacheBasePath);
+
+  bool load();
+  [[nodiscard]] const std::string& getPath() const { return filepath; }
+  [[nodiscard]] const std::string& getCachePath() const { return cachePath; }
+  [[nodiscard]] std::string getTitle() const;
+  [[nodiscard]] size_t getFileSize() const { return fileSize; }
+
+  void setupCacheDir() const;
+
+  // Cover image support - looks for cover.bmp/jpg/jpeg/png in same folder as txt file
+  [[nodiscard]] std::string getCoverBmpPath() const;
+  [[nodiscard]] bool generateCoverBmp() const;
+  [[nodiscard]] std::string findCoverImage() const;
+
+  // Read content from file
+  [[nodiscard]] bool readContent(uint8_t* buffer, size_t offset, size_t length) const;
+};