Add XTC/XTCH ebook format support (#135)

## Summary

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

Add support for XTC (XTeink X4 native) ebook format, which contains
pre-rendered 480x800 1-bit bitmap pages optimized for e-ink displays.

* **What changes are included?**

- New `lib/Xtc/` library with XtcParser for reading XTC files
- XtcReaderActivity for displaying XTC pages on e-ink display
- XTC file detection in FileSelectionActivity
- Cover BMP generation from first XTC page
- Correct XTG page header structure (22 bytes) and bit polarity handling

## Additional Context

- XTC files contain pre-rendered bitmap pages with embedded status bar
(page numbers, progress %)
- XTG page header: 22 bytes (magic + dimensions + reserved fields +
bitmap size)
- Bit polarity: 0 = black, 1 = white
- No runtime text rendering needed - pages display directly on e-ink
- Faster page display compared to EPUB since no parsing/rendering
required
- Memory efficient: loads one page at a time (48KB per page)
- Tested with XTC files generated from https://x4converter.rho.sh/
- Verified correct page alignment and color rendering
- Please report any issues if you test with XTC files from other
sources.

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/activities/boot_sleep/SleepActivity.cpp

@@ -4,6 +4,7 @@
 #include <FsHelpers.h>
 #include <GfxRenderer.h>
 #include <SD.h>
+#include <Xtc.h>
 
 #include <vector>
 
@@ -12,6 +13,20 @@
 #include "config.h"
 #include "images/CrossLarge.h"
 
+namespace {
+// Check if path has XTC extension (.xtc or .xtch)
+bool isXtcFile(const std::string& path) {
+  if (path.length() < 4) return false;
+  std::string ext4 = path.substr(path.length() - 4);
+  if (ext4 == ".xtc") return true;
+  if (path.length() >= 5) {
+    std::string ext5 = path.substr(path.length() - 5);
+    if (ext5 == ".xtch") return true;
+  }
+  return false;
+}
+}  // namespace
+
 void SleepActivity::onEnter() {
   Activity::onEnter();
   renderPopup("Entering Sleep...");
@@ -176,19 +191,41 @@ void SleepActivity::renderCoverSleepScreen() const {
     return renderDefaultSleepScreen();
   }
 
-  Epub lastEpub(APP_STATE.openEpubPath, "/.crosspoint");
-  if (!lastEpub.load()) {
-    Serial.println("[SLP] Failed to load last epub");
-    return renderDefaultSleepScreen();
-  }
+  std::string coverBmpPath;
 
-  if (!lastEpub.generateCoverBmp()) {
-    Serial.println("[SLP] Failed to generate cover bmp");
-    return renderDefaultSleepScreen();
+  // Check if the current book is XTC or EPUB
+  if (isXtcFile(APP_STATE.openEpubPath)) {
+    // Handle XTC file
+    Xtc lastXtc(APP_STATE.openEpubPath, "/.crosspoint");
+    if (!lastXtc.load()) {
+      Serial.println("[SLP] Failed to load last XTC");
+      return renderDefaultSleepScreen();
+    }
+
+    if (!lastXtc.generateCoverBmp()) {
+      Serial.println("[SLP] Failed to generate XTC cover bmp");
+      return renderDefaultSleepScreen();
+    }
+
+    coverBmpPath = lastXtc.getCoverBmpPath();
+  } else {
+    // Handle EPUB file
+    Epub lastEpub(APP_STATE.openEpubPath, "/.crosspoint");
+    if (!lastEpub.load()) {
+      Serial.println("[SLP] Failed to load last epub");
+      return renderDefaultSleepScreen();
+    }
+
+    if (!lastEpub.generateCoverBmp()) {
+      Serial.println("[SLP] Failed to generate cover bmp");
+      return renderDefaultSleepScreen();
+    }
+
+    coverBmpPath = lastEpub.getCoverBmpPath();
   }
 
   File file;
-  if (FsHelpers::openFileForRead("SLP", lastEpub.getCoverBmpPath(), file)) {
+  if (FsHelpers::openFileForRead("SLP", coverBmpPath, file)) {
     Bitmap bitmap(file);
     if (bitmap.parseHeaders() == BmpReaderError::Ok) {
       renderBitmapSleepScreen(bitmap);