Add TXT file reader support (#240)

## Summary

* **What is the goal of this PR?** 

Add support for reading plain text (.txt) files, enabling users to
browse, read, and track progress in TXT documents alongside existing
EPUB and XTC formats.

* **What changes are included?**

- New Txt library for loading and parsing plain text files
- New TxtReaderActivity with streaming page rendering using 8KB chunks
to handle large files without memory issues on ESP32-C3
- Page index caching system (index.bin) for instant re-open after sleep
or app restart
- Progress bar UI during initial file indexing (matching EPUB style)
- Word wrapping with proper UTF-8 support
- Cover image support for TXT files:
- Primary: image with same filename as TXT (e.g., book.jpg for book.txt)
  - Fallback: cover.bmp/jpg/jpeg in the same folder
  - JPG to BMP conversion using existing converter
  - Sleep screen cover mode now works with TXT files
- File browser now shows .txt files

## Additional Context

* Add any other information that might be helpful for the reviewer

* Memory constraints: The streaming approach was necessary because
ESP32-C3 only has 320KB RAM. A 700KB TXT file cannot be loaded entirely
into memory, so we read 8KB chunks and build a page offset index
instead.

* Cache invalidation: The page index cache automatically invalidates
when file size, viewport width, or lines per page changes (e.g., font
size or orientation change).

* Performance: First open requires indexing (with progress bar),
subsequent opens load from cache instantly.

* Cover image format: PNG is detected but not supported for conversion
(no PNG decoder available). Only BMP and JPG/JPEG work.

src/activities/reader/ReaderActivity.h

@@ -5,6 +5,7 @@
 
 class Epub;
 class Xtc;
+class Txt;
 
 class ReaderActivity final : public ActivityWithSubactivity {
   std::string initialBookPath;
@@ -12,13 +13,16 @@ class ReaderActivity final : public ActivityWithSubactivity {
   const std::function<void()> onGoBack;
   static std::unique_ptr<Epub> loadEpub(const std::string& path);
   static std::unique_ptr<Xtc> loadXtc(const std::string& path);
+  static std::unique_ptr<Txt> loadTxt(const std::string& path);
   static bool isXtcFile(const std::string& path);
+  static bool isTxtFile(const std::string& path);
 
   static std::string extractFolderPath(const std::string& filePath);
   void onSelectBookFile(const std::string& path);
   void onGoToFileSelection(const std::string& fromBookPath = "");
   void onGoToEpubReader(std::unique_ptr<Epub> epub);
   void onGoToXtcReader(std::unique_ptr<Xtc> xtc);
+  void onGoToTxtReader(std::unique_ptr<Txt> txt);
 
  public:
   explicit ReaderActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, std::string initialBookPath,