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/boot_sleep/SleepActivity.cpp

@@ -3,6 +3,7 @@
 #include <Epub.h>
 #include <GfxRenderer.h>
 #include <SDCardManager.h>
+#include <Txt.h>
 #include <Xtc.h>
 
 #include "CrossPointSettings.h"
@@ -207,6 +208,7 @@ void SleepActivity::renderCoverSleepScreen() const {
   std::string coverBmpPath;
   bool cropped = SETTINGS.sleepScreenCoverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP;
 
+  // Check if the current book is XTC, TXT, or EPUB
   if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtc") ||
       StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtch")) {
     // Handle XTC file
@@ -222,6 +224,20 @@ void SleepActivity::renderCoverSleepScreen() const {
     }
 
     coverBmpPath = lastXtc.getCoverBmpPath();
+  } else if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".txt")) {
+    // Handle TXT file - looks for cover image in the same folder
+    Txt lastTxt(APP_STATE.openEpubPath, "/.crosspoint");
+    if (!lastTxt.load()) {
+      Serial.println("[SLP] Failed to load last TXT");
+      return renderDefaultSleepScreen();
+    }
+
+    if (!lastTxt.generateCoverBmp()) {
+      Serial.println("[SLP] No cover image found for TXT file");
+      return renderDefaultSleepScreen();
+    }
+
+    coverBmpPath = lastTxt.getCoverBmpPath();
   } else if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".epub")) {
     // Handle EPUB file
     Epub lastEpub(APP_STATE.openEpubPath, "/.crosspoint");