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>

lib/GfxRenderer/GfxRenderer.cpp

@@ -341,12 +341,13 @@ void GfxRenderer::freeBwBufferChunks() {
  * This should be called before grayscale buffers are populated.
  * A `restoreBwBuffer` call should always follow the grayscale render if this method was called.
  * Uses chunked allocation to avoid needing 48KB of contiguous memory.
+ * Returns true if buffer was stored successfully, false if allocation failed.
  */
-void GfxRenderer::storeBwBuffer() {
+bool GfxRenderer::storeBwBuffer() {
   const uint8_t* frameBuffer = einkDisplay.getFrameBuffer();
   if (!frameBuffer) {
     Serial.printf("[%lu] [GFX] !! No framebuffer in storeBwBuffer\n", millis());
-    return;
+    return false;
   }
 
   // Allocate and copy each chunk
@@ -367,7 +368,7 @@ void GfxRenderer::storeBwBuffer() {
                     BW_BUFFER_CHUNK_SIZE);
       // Free previously allocated chunks
       freeBwBufferChunks();
-      return;
+      return false;
     }
 
     memcpy(bwBufferChunks[i], frameBuffer + offset, BW_BUFFER_CHUNK_SIZE);
@@ -375,6 +376,7 @@ void GfxRenderer::storeBwBuffer() {
 
   Serial.printf("[%lu] [GFX] Stored BW buffer in %zu chunks (%zu bytes each)\n", millis(), BW_BUFFER_NUM_CHUNKS,
                 BW_BUFFER_CHUNK_SIZE);
+  return true;
 }
 
 /**
@@ -422,6 +424,17 @@ void GfxRenderer::restoreBwBuffer() {
   Serial.printf("[%lu] [GFX] Restored and freed BW buffer chunks\n", millis());
 }
 
+/**
+ * Cleanup grayscale buffers using the current frame buffer.
+ * Use this when BW buffer was re-rendered instead of stored/restored.
+ */
+void GfxRenderer::cleanupGrayscaleWithFrameBuffer() const {
+  uint8_t* frameBuffer = einkDisplay.getFrameBuffer();
+  if (frameBuffer) {
+    einkDisplay.cleanupGrayscaleBuffers(frameBuffer);
+  }
+}
+
 void GfxRenderer::renderChar(const EpdFontFamily& fontFamily, const uint32_t cp, int* x, const int* y,
                              const bool pixelState, const EpdFontStyle style) const {
   const EpdGlyph* glyph = fontFamily.getGlyph(cp, style);