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/TxtReaderActivity.h

@@ -0,0 +1,60 @@
+#pragma once
+
+#include <Txt.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <vector>
+
+#include "CrossPointSettings.h"
+#include "activities/ActivityWithSubactivity.h"
+
+class TxtReaderActivity final : public ActivityWithSubactivity {
+  std::unique_ptr<Txt> txt;
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  int currentPage = 0;
+  int totalPages = 1;
+  int pagesUntilFullRefresh = 0;
+  bool updateRequired = false;
+  const std::function<void()> onGoBack;
+  const std::function<void()> onGoHome;
+
+  // Streaming text reader - stores file offsets for each page
+  std::vector<size_t> pageOffsets;  // File offset for start of each page
+  std::vector<std::string> currentPageLines;
+  int linesPerPage = 0;
+  int viewportWidth = 0;
+  bool initialized = false;
+
+  // Cached settings for cache validation (different fonts/margins require re-indexing)
+  int cachedFontId = 0;
+  int cachedScreenMargin = 0;
+  uint8_t cachedParagraphAlignment = CrossPointSettings::LEFT_ALIGN;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void renderScreen();
+  void renderPage();
+  void renderStatusBar(int orientedMarginRight, int orientedMarginBottom, int orientedMarginLeft) const;
+
+  void initializeReader();
+  bool loadPageAtOffset(size_t offset, std::vector<std::string>& outLines, size_t& nextOffset);
+  void buildPageIndex();
+  bool loadPageIndexCache();
+  void savePageIndexCache() const;
+  void saveProgress() const;
+  void loadProgress();
+
+ public:
+  explicit TxtReaderActivity(GfxRenderer& renderer, MappedInputManager& mappedInput, std::unique_ptr<Txt> txt,
+                             const std::function<void()>& onGoBack, const std::function<void()>& onGoHome)
+      : ActivityWithSubactivity("TxtReader", renderer, mappedInput),
+        txt(std::move(txt)),
+        onGoBack(onGoBack),
+        onGoHome(onGoHome) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};