Add chapter select support to XTC files (#145)

## Summary

- **What is the goal of this PR?** Add chapter selection support to the
XTC reader activity, including parsing chapter metadata from XTC files.
- **What changes are included?** Implemented XTC chapter parsing and
exposure in the XTC library, added a chapter selection activity for XTC,
integrated it into XtcReaderActivity, and normalized chapter page
indices by shifting them to 0-based.

  ## Additional Context

- The reader uses 0-based page indexing (first page = 0), but the XTC
chapter table appears to be 1-based (first page = 1), so chapter
start/end pages are shifted down by 1 during parsing.

lib/Xtc/Xtc.cpp

@@ -87,6 +87,21 @@ std::string Xtc::getTitle() const {
   return filepath.substr(lastSlash, lastDot - lastSlash);
 }
 
+bool Xtc::hasChapters() const {
+  if (!loaded || !parser) {
+    return false;
+  }
+  return parser->hasChapters();
+}
+
+const std::vector<xtc::ChapterInfo>& Xtc::getChapters() const {
+  static const std::vector<xtc::ChapterInfo> kEmpty;
+  if (!loaded || !parser) {
+    return kEmpty;
+  }
+  return parser->getChapters();
+}
+
 std::string Xtc::getCoverBmpPath() const { return cachePath + "/cover.bmp"; }
 
 bool Xtc::generateCoverBmp() const {