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.

src/activities/reader/XtcReaderChapterSelectionActivity.cpp

@@ -0,0 +1,152 @@
+#include "XtcReaderChapterSelectionActivity.h"
+
+#include <GfxRenderer.h>
+#include <InputManager.h>
+#include <SD.h>
+
+#include "config.h"
+
+namespace {
+constexpr int SKIP_PAGE_MS = 700;
+}  // namespace
+
+int XtcReaderChapterSelectionActivity::getPageItems() const {
+  constexpr int startY = 60;
+  constexpr int lineHeight = 30;
+
+  const int screenHeight = renderer.getScreenHeight();
+  const int availableHeight = screenHeight - startY;
+  int items = availableHeight / lineHeight;
+  if (items < 1) {
+    items = 1;
+  }
+  return items;
+}
+
+int XtcReaderChapterSelectionActivity::findChapterIndexForPage(uint32_t page) const {
+  if (!xtc) {
+    return 0;
+  }
+
+  const auto& chapters = xtc->getChapters();
+  for (size_t i = 0; i < chapters.size(); i++) {
+    if (page >= chapters[i].startPage && page <= chapters[i].endPage) {
+      return static_cast<int>(i);
+    }
+  }
+  return 0;
+}
+
+void XtcReaderChapterSelectionActivity::taskTrampoline(void* param) {
+  auto* self = static_cast<XtcReaderChapterSelectionActivity*>(param);
+  self->displayTaskLoop();
+}
+
+void XtcReaderChapterSelectionActivity::onEnter() {
+  Activity::onEnter();
+
+  if (!xtc) {
+    return;
+  }
+
+  renderingMutex = xSemaphoreCreateMutex();
+  selectorIndex = findChapterIndexForPage(currentPage);
+
+  updateRequired = true;
+  xTaskCreate(&XtcReaderChapterSelectionActivity::taskTrampoline, "XtcReaderChapterSelectionActivityTask",
+              4096,               // Stack size
+              this,               // Parameters
+              1,                  // Priority
+              &displayTaskHandle  // Task handle
+  );
+}
+
+void XtcReaderChapterSelectionActivity::onExit() {
+  Activity::onExit();
+
+  xSemaphoreTake(renderingMutex, portMAX_DELAY);
+  if (displayTaskHandle) {
+    vTaskDelete(displayTaskHandle);
+    displayTaskHandle = nullptr;
+  }
+  vSemaphoreDelete(renderingMutex);
+  renderingMutex = nullptr;
+}
+
+void XtcReaderChapterSelectionActivity::loop() {
+  const bool prevReleased = mappedInput.wasReleased(MappedInputManager::Button::Up) ||
+                            mappedInput.wasReleased(MappedInputManager::Button::Left);
+  const bool nextReleased = mappedInput.wasReleased(MappedInputManager::Button::Down) ||
+                            mappedInput.wasReleased(MappedInputManager::Button::Right);
+
+  const bool skipPage = mappedInput.getHeldTime() > SKIP_PAGE_MS;
+  const int pageItems = getPageItems();
+
+  if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
+    const auto& chapters = xtc->getChapters();
+    if (!chapters.empty() && selectorIndex >= 0 && selectorIndex < static_cast<int>(chapters.size())) {
+      onSelectPage(chapters[selectorIndex].startPage);
+    }
+  } else if (mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+    onGoBack();
+  } else if (prevReleased) {
+    const int total = static_cast<int>(xtc->getChapters().size());
+    if (total == 0) {
+      return;
+    }
+    if (skipPage) {
+      selectorIndex = ((selectorIndex / pageItems - 1) * pageItems + total) % total;
+    } else {
+      selectorIndex = (selectorIndex + total - 1) % total;
+    }
+    updateRequired = true;
+  } else if (nextReleased) {
+    const int total = static_cast<int>(xtc->getChapters().size());
+    if (total == 0) {
+      return;
+    }
+    if (skipPage) {
+      selectorIndex = ((selectorIndex / pageItems + 1) * pageItems) % total;
+    } else {
+      selectorIndex = (selectorIndex + 1) % total;
+    }
+    updateRequired = true;
+  }
+}
+
+void XtcReaderChapterSelectionActivity::displayTaskLoop() {
+  while (true) {
+    if (updateRequired) {
+      updateRequired = false;
+      xSemaphoreTake(renderingMutex, portMAX_DELAY);
+      renderScreen();
+      xSemaphoreGive(renderingMutex);
+    }
+    vTaskDelay(10 / portTICK_PERIOD_MS);
+  }
+}
+
+void XtcReaderChapterSelectionActivity::renderScreen() {
+  renderer.clearScreen();
+
+  const auto pageWidth = renderer.getScreenWidth();
+  const int pageItems = getPageItems();
+  renderer.drawCenteredText(READER_FONT_ID, 10, "Select Chapter", true, BOLD);
+
+  const auto& chapters = xtc->getChapters();
+  if (chapters.empty()) {
+    renderer.drawCenteredText(UI_FONT_ID, 120, "No chapters", true, REGULAR);
+    renderer.displayBuffer();
+    return;
+  }
+
+  const auto pageStartIndex = selectorIndex / pageItems * pageItems;
+  renderer.fillRect(0, 60 + (selectorIndex % pageItems) * 30 - 2, pageWidth - 1, 30);
+  for (int i = pageStartIndex; i < static_cast<int>(chapters.size()) && i < pageStartIndex + pageItems; i++) {
+    const auto& chapter = chapters[i];
+    const char* title = chapter.name.empty() ? "Unnamed" : chapter.name.c_str();
+    renderer.drawText(UI_FONT_ID, 20, 60 + (i % pageItems) * 30, title, i != selectorIndex);
+  }
+
+  renderer.displayBuffer();
+}