fix: Defer low-power mode during section indexing and book loading

Prevent the device from dropping to 10MHz CPU during first-time chapter
indexing, cover prerendering, and other CPU-intensive reader operations.

Three issues addressed:
- ActivityWithSubactivity now delegates preventAutoSleep() and
  skipLoopDelay() to the active subactivity, so EpubReaderActivity's
  signal is visible through the ReaderActivity wrapper
- Added post-loop() re-check of preventAutoSleep() in main.cpp to
  catch activity transitions that happen mid-loop
- EpubReaderActivity uses both !section and a loadingSection flag to
  cover the full duration from activity entry through section file
  creation; TxtReaderActivity uses !initialized similarly

Also syncs HalPowerManager.cpp log messages with upstream PR #852.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/activities/reader/EpubReaderActivity.cpp

@@ -861,6 +861,8 @@ void EpubReaderActivity::renderScreen() {
   }
 
   if (!section) {
+    loadingSection = true;
+
     const auto filepath = epub->getSpineItem(currentSpineIndex).href;
     LOG_DBG("ERS", "Loading file: %s, index: %d", filepath.c_str(), currentSpineIndex);
     section = std::unique_ptr<Section>(new Section(epub, currentSpineIndex, renderer));
@@ -880,6 +882,7 @@ void EpubReaderActivity::renderScreen() {
                                       viewportHeight, SETTINGS.hyphenationEnabled, SETTINGS.embeddedStyle, popupFn)) {
         LOG_ERR("ERS", "Failed to persist page data to SD");
         section.reset();
+        loadingSection = false;
         return;
       }
     } else {
@@ -912,6 +915,8 @@ void EpubReaderActivity::renderScreen() {
       section->currentPage = newPage;
       pendingPercentJump = false;
     }
+
+    loadingSection = false;
   }
 
   renderer.clearScreen();